Probably for alignment. You want the class pointer (whether 32- or 64-bit) to be easy to dereference with little or no preprocessing. It looks like the HotSpot

Probably for alignment. You want the class pointer (whether 32- or 64-bit) to be easy to dereference with little or no preprocessing. It looks like the HotSpot JVM implementers decided to use 5 bits for lock and GC, and 31 bits for identity hash code (not sure why they didn't use 32). That adds up to 36 bits. You'd want to round this up to a multiple of 32 bits so that both the beginning of the object and the class pointer are aligned to 32 or 64 bits.