C++ log() and log10()

log (or written commonly as ln) has a great significance in the world of mathematics, however, for computer implementation on some specific cases, I am not sure which one is better.

Yesterday, I was trying to solve a problem from uva online judge where I needed to find log_ba, which is calculated as log(a) / log(b) in C++. However, doing so led to a "wrong answer". Then, just to check, rewrote that formula with an equivalent one using log10(a) / log10(b), and this time it worked fine. Really confusing, so I come to this conclusion, log (e based) is not quite precise on computer implementation, especially with gcc/g++. Because, both produces same output on my machine, but different in uva's gcc/g++ compiler. Any other reason?