round() function strange behaviour

[brobbins333 at shaw.ca]

the round() function will sometimes return a predictable result, as
in:

round(45.67891, 2)
>>> 45.68

but sometimes it will return something like:

round(56.78923, 2)
>>> 56.789999999999999

2.1 and 2.2 both behave this way on two different (Windows) computers.
What's going on here?

Bruce Robbins