The description of the "Format Specification Mini-Language" states for float and Decimal regarding presentation type 'f':
"The default precision is 6."
Regarding presentation type None it reads:
"Similar to 'g', except that fixed-point notation, when used, has at least one digit past the decimal point."
While both statements are accurate for float, they don't hold for Decimal.
In order to preserve the information about the decimal exponent, in both cases Decimal formatting displays as many fractional digits as dictated by it's exponent.