Bonuses from Maker

The range of any given stat is:
base +/-floor(base/10)

For normal drops, there is a cap at +/- 5.
For Gachapon, the cap is at +7 (not sure about -).

Gachapon can go -7 too.

And it's +- floor(base/10 + 1) for Gacha - that's why PACs range from 0-4 atk instead of 1-3.

For normal equips the range is -5 to +5 but they're not all equally likely. In fact 0 is the most common result, and +-5 are the least common. If you've spent any time looking at the stats of equip drops, you'll know this - +5 weapons are extremely rare. So it's not really a surprise that even with Stimulators in play, you get average equips more often than well above average ones.