Cash Shop Specials: 1/1 - 1/8

I don't think so. It's just that slots from the CS hammers can be reversed while slots added from event hammers can't so it might be the latter case.

The game can't tell what kind of hammer you got your slot from. Or which slot you failed a scroll on.
All it knows is whether the + is less than the maximum it could be, considering the # of slots that item has when clean, the "upgrades available" and the "number of hammer applied".

So for example if I have a regular weapon (7 slots when clean), and it says:

(+6)
upgrades available: 0
hammers applied: 1

Then the game knows I've failed two slots (it could have been +8 by now, since I used a hammer).
It can't possibly know whether I failed the first two, the last two, the 4th and 8th, or any others.
It also can't know whether either of the failed slots was due to a 50% hammer, or both of them due to normal scrolls failing.
All it knows and all it cares is that there are two failed slots. If the clean slate works, it will restore one of them, so the weapon will show:

(+6)
upgrades available: 1
hammers applied: 1