Redis I calculate size of key is different from memory usage size of key, Why?

I execute "set a zhangsan EX 10000". Then, I get memory size by "memory usage a", it is 55. I kill redis-server to generate rdb file. I add some code in "rdbLoadRio" function: `

    size_t key_usage = sdsAllocSize(val->ptr);

    size_t val_usage = objectComputeSize(val, 5);

    size_t dict_entry_usage = sizeof(dictEntry);

    size_t usage = key_usage + val_usage + dict_entry_usage;

` The useage is not equal to 55. The usage is 53, Why?

Note: (1)Redis server v=999.999.999 sha=35abf9ed:1 malloc=jemalloc-4.0.3 bits=64 build=2b0a01d0af5997ed

Comment From: yoav-steinberg

This doesn't recreate. I'm closing this. If still relevant try to recreate with latest version, let me know, and we'll re-open.