You have might other processes running on the principle server which are not running the mirror server. This way there is more memory for SQL Server on the mirror to take and none to give back to. Doesn't seem to be problem to me.

Could be a coincidence that you noticed this behavior started when the disk was replaced. Are you sure this was not happening before? Do you have an automatic memory usage monitor which keeps a history?

As long you don't have problems I wouldn't spend a lot of time in this.