Just to clarify:

If not specified, the default "presentation type" for floats is "g". Since you didn't specify "f" in your example, it's using "g". For presentation type "g", the precision (3 in your case) is the total number of significant digits. So 12.34 first becomes 12.3, then that's displayed in a field 8 characters wide, which gives you the 4 leading zeros.

See the section of presentation types for floating point at the end of this section:
https://docs.python.org/2/library/string.html#format-specification-mini-language