This is totally normal. Your NBPEL is going to change every day depending on the conditions. The only true measurement you can make is bone pressed. When you anchor against the bone you’re getting a true reading but if you’re not anchored against the bone you’re fat pad is going to be the issue.
The best way to convert that size is to do erect stretching. The more the better as this stress is erectile tissue as opposed to normal stretching. Give it a shot let me know how it goes