Number Needed to treat is the number of patients you needed to treat, so that one patient can get the benifit of the drug. That is, if on experiments 1% got a better result from the drug than placebo (ARR=1%), if you elimenate the effect of chance, you need to treat a 100 patients so that one would get the benifit (be the 1%). This is obtained from 100/1, inverted ARR.
Number Needed to Treat for harm has the same concept, but instead of ARR, it is absolute harm increase, which's the incidence difference of an adverse effect between placebo and the those who took the drug.
So at least UW is more accurate, but limited to number needed to treat to benifit (NNTB, or NNT), I don't know what incidence Kaplan means, but maybe it's either ARR or absolute harm increase..