Sorry for misreading your question.

It's (again) a floating point problem.

  DB<149> $a=14.4
 => "14.4"

  DB<150> $b=10+$a-10
 => "14.4"

  DB<151> $a-$b
 => "1.77635683940025e-15"
[download]

The mantissa does loose some digits at the end after adding 10 and substracting doesn't bring them back.

A proper test is either to stay integer from the beginning till the end or to calculate the difference and to compare against a threshold (trickier).

Please note how 14.5 doesn't have this problem since 1/2 is a power of 2 (i.e. only 0s are lost when shifting the mantissa)

update

see Re: eq vs == and Humans have too many fingers for more details