Heaps "slightly below 32GB" are usually because of the -XX:+UseCompressedOops option, which allows Java to address up to 32GB of memory with a smaller pointer.

Heaps "slightly below 32GB" are usually because of the -XX:+UseCompressedOops option, which allows Java to address up to 32GB of memory with a smaller pointer. Between 32-35GB of heap, you're just paying off the savings you would have gotten with compressed object pointers, but if you keep cranking your heap further after that, you'll start getting benefits again.