irb> 4.4-4.0
=> 0.4
irb> 8.4-8.0
=> 0.4
irb> 16.4-16.0
=> 0.399999999999999
irb> 32.4-32.0
=> 0.399999999999999
irb> 64.4-64.0
=> 0.400000000000006
irb> 128.4-128.0
=> 0.400000000000006
irb> 128.4-127.0
=> 1.40000000000001
irb> 128.4-126.0
=> 2.40000000000001
irb> 128.4-125.0
=> 3.40000000000001
irb> 128.4-124.0
=> 4.40000000000001
irb> 128.4-123.0
=> 5.40000000000001
irb> 128.4-121.0
=> 7.40000000000001
irb> 128.4-120.0
=> 8.40000000000001
irb> 128.4-119.0
=> 9.40000000000001
irb> 128.4-118.0
=> 10.4

確かに桁丸めとか桁落ちとかなんかあのへんのめんどくさいやつの問題っぽいな