performance difference between c++ 4.3.2 and cpp 6.3 compiler

While i was solving problems on SPOJ i found cpp compiler c++4.3.2 is memory efficient(takes less memory) while cpp 6.3 time efficient(takes less time)for same solution.Is this true and why?

+ PROBLEM          TIME    MEM     LANG
+ Prime Interval    0.18    15M     CPP
+ Prime Interval    0.26    2.8M    C++ 4.3.2