Your workaround tells the whole story:
(/ 1 2) ;=> 0
(/ 1.0 2) ;=>0.5
With integers, emacs does integer arithmetic, with floats it does floating-point arithmetic.
See the Elisp manual, node Arithmetic Operations:
%, each of these functions accepts both integer and
floating-point arguments, and returns a floating-point number if any
argument is floating point.
If all the arguments are integers, the result is an integer,
obtained by rounding the quotient towards zero after each division.
See also node Numeric Conversions, for info about explicitly converting among numeric types.