>>22314880>>22314978It actually multiplies your damage by 3/2.
People just multiply the BP for convenience. This only gives a decent representation at higher levels though. At lv100 a neutral STAB Thunderbolt would be stronger than a non-STAB 2* HP (90*3/2 > 60*2 in the common BP notation) but at Lv5 this isn't the case, pic related.