KSC USP Compact Mag capacity issue?

[Fulcrum]

I have owned my USPc since January (review coming soon.) and it's been great. I have just purchased a second mag.
The original mag on a full charge (134) will give me 3 1/2 reloads before I have to refill. The new mag will give me 2 1/2 reloads before a refill.
As far as I can tell, the mags are exactly identical except for the fit in the frame. The new mag is slightly loose but this does not seem to affect the performance at all.
Are there different mag capacities? I've not heard of this before.

Any help would be appreciated.

[Steve_U1S]

I fixed this for him - turns out the fill valve might be 'sticky' or something; I popped in one from a 'parts' mag I have for a KSC G-series, and it brought it up to par with the other one.