0.255 and 1.255 are not exactly representable in base-2 floating point:

$ perl -we 'printf "%.20f\n", 0.255'
0.25500000000000000444
$ perl -we 'printf "%.20f\n", 1.255'
1.25499999999999989342
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when the internal represenations of these two numbers are then rounded to 2 decimal places, one rounds up, the other down.

Dave.