I will keep it simple, but here is an example of a 90% confidence interval on a 9/6 Jacks or better Video Poker played mathematically perfectly. The same can be said for slot play although at a much lower payback range, substituting number of spins for number of hands. So this gives you an example of why we can have wild swings and how you would really need about 1,000,000 hand/spins to approach any advertised "payback" range... Slotspert

Number of Hands.... Payback intervals
1,000.....................76.5%-122.6%
10,000...................92.3%-106.8%
100,000................. 97.2%-101.8%
1,000,000.............. 98.8%-100.3%
10,000,000............. 99.3%-99.8%
100,000,000........... 99.5%-99.6%
1,000,000,000......... 99.5%-99.6%

P.S. For your brain's comfort, slots spins are always "played" perfectly whereas video poker hands are usually not.