At my last adjustment my Orthodontist had an impression/mould taken of my lower teeth and informed me that at my next appointment in 4weeks he would be putting on my lower permanent retainer but be leaving the lower braces on for another month

He didn't mention when my upper braces would be coming off (there's still work to be done on my bite and positioning of my upper incisors).
My question is why would he go ahead and put in my lower permanent retainer but leave the lower braces on for another month (challenges to clean there

Puzzled and would appreciate input on this subject!