I have no idea where that 2260 comes from because it is shown in some advertisements and tables but not from Hornady directly nor from their own website.
Here's a page on an ammo supplier's website. The line just above the ammo box picture shows
2260 and it even includes the mfg item # 8585
Right below it is the manufacturer's (Hornady) specs.- the specs show
2140 and includes the mfg item # 8585
Still haven't seen any direct chronograph results of this particular Hornady 500 gr 458 WM ammo ... a more or less valid, posted result would be something like: complete description of type of chronograph and set up. Then give the number of shots, mean velocity and standard deviation of velocity. Easy to find standard deviation calculators on the net. I would think 5-10 shots would be adequate.
Go to Hornady website, link below. Click on 458 WM 500 gr DGS Superformance ammo. When page opens look at specs
https://www.hornady.com/ammunition/rifle/#!/