This is actually true of the CZ 550 .375 H&H cartridge family as well, but there is a twist to it
1- Most often, depending on the metal bottom inleting in the stock, the magazine can indeed hold 6 rounds, but with 6 rounds in the mag, the first round will often jam because there is not enough depression space left in the magazine to allow the cartridge to feed smoothly, with its butt tilting down slightly as its nose tilts up. You want to check on your own rifle if that sixth round feeds from the mag. Some rifles will feed, some will not. It depends on how deep in the stock the floor plate is inletted.
2- This being said, the actual purpose of having more room in the magazine than needed for 5 rounds is that you can put a sixth round of top of the five, without inserting it in the magazine, then depress the sixth round halfway in the mag as you move the bolt forward so that the round can engage under the extractor, and you can close the bolt on the sixth round without having to snap the extractor over it. It you snap the extractor too often over a round that you just pushed into the chamber, ultimately you will break the extractor. The "miraculous and unexpected bonus" is not one actually, it is the unintended consequence of the proper designing of the system to be able to load five cartridges in the mag, then one in the chamber, the proper way, without snapping the extractor.
Originally, Mauser extractors were not beveled, and could not snap over a cartridge head (which defeats one of the major advantages of such extractors: the fact that they could not snap over a cartridge head, therefore extraction was virtually guaranteed.) but probably too many folks complained about their bolt not being able to close with a cartridge in the chamber, so nowadays most Mauser extractors, including the CZ, are beveled. This is a marketing compromise to please ill-informed customers, but an actual downgrade of the original Mauser design...
I hope this was of interest
In the end, the CZ 550 is the same action, built on the same machinery, in the same plant as the ZKK 602 was. The plant used the ZKK brand before the collapse of the iron curtain and the CZ brand after.
Aside from minor evolution (safety, bolt handle shape, etc.) the two actions are identical. Admittedly, some series of CZ 550 suffer from very poor machining (when the tooling wears out I would speculate). My .416 Rigby needed significant deburring to become smooth. Conversely, other series of CZ 550 are clean (when cutting tools are periodically replaced, I would speculate). My .375 H&H was smooth from the get go.
The only advantages the ZKK-period rifles have is that there clearly was an additional manufacturing step then: manual deburring. Labor was cheap under communist rule, I guess... This step has obviously been removed under new-to-them capitalist rule.
I suspect in the end that this may have been a very ill-advised costs saving step, because it allowed the "CZ do not work out the box" legend to start. It is false in most cases, but true in enough cases apparently... I have personally never seen a factory CZ that did not work - I have seen very rough ones, but this is different - but credible people have reported it. Nothing that cannot be fixed in a few hours with a deburring tool and some fine grit sand paper, but in the land of American instant gratification, it has proved a brand killer...
In the end, the CZ 550 gets compared to the "Win 70 that works out of the box" and this is a complete misunderstanding of what one gets with a CZ 550 which is true magnum length square bridge action capable of housing .416 Rigby length cartridges and offering built in scope bases. Try to build a .416 Rigby and to weld scope bases on a Win 70 action
The rumor is that CZ will soon stop making the 550 actions entirely. Let's see how long it takes for them to develop cult status once people realize that the only other options are Mauser, Granite Mountain, etc. $6,000 actions (not rifles, mind you, actions alone!). Because these are the actions the CZ 550 should be mechanically compared too, not the Win 70
It is not entirely by chance that Rigby used the ZKK / CZ action to build their $15,000 Rigby rifles for decades before Mauser re-introduced the magnum length action
In conclusion, the ZKK 602 is a great rifle. Long live the ZKK indeed! Consider it an already deburred CZ 550 ... and use that extra mag space correctly to load the sixth cartridge in the chamber without snapping the extractor