Some specifics about the numbers you provided:

>perl -le"printf qq{%.16e\n}, 5.1"
5.0999999999999996e+000

>perl -le"printf qq{%.16e\n}, 100005.2"
1.0000520000000000e+005
[download]

Notice how the computer is unable to represent 5.1 exactly? Others have explained why.