Standards compliant means it implements the standard. Mac OS X is POSIX compliant even though it has an entire universe of APIs besides POSIX. No POSIX-compliant OS just implements POSIX. Web standards are much the same. Good browser developers try not to pollute the namespace with browser-specific extensions though (ie: prefix with moz or webkit).
The point of standards is that there is a common baseline that just works. If I write standard compliant code I expert it to work on all standard compliant platforms (if that be compilers, OSes, browsers, etc.). This is very helpful for portability, having the baseline with clearly defined boundaries. But it does not mean every platform has to be bit-for-bit the same interface.
