factor
does not mean the same thing in Z (or Q) and in SR
: if, on both cases, factor
aims at (recursively) transforming its input in a product of "simpler" elements, the meaning of "simpler" differs:
In Z (or Q), the "simplest" elements are primes;
in SR
, they are (roughly) polynomials.
In SR
, 6 is already a monomial, hence a non-simplifiable element, whereas, in Z, it is not a prime, hence factorizable as a product of primes.
Another illustration:
sage: factor(6/5)
2 * 3 * 5^-1
sage: factor(SR(6/5))
6/5
HTH,