>>gorbak+U4
Depends on if they're using smart memory allocation to keep costs lower, IE, if they can pattern that certain workloads only need N amount of memory at Y time, they can effectively borrow memory from one VM for usage in another that has an opposite statistical likelihood of needing that memory.
This is why paying for dedicated memory is often more expensive than its counter part, because that dedicated memory is not considered as part of pooling.