Plumbing question - Blue polybutylene

In doing a re-plumb for a mobile home that originally used the blue polybutylene 3/4 water line, I notice that a connection just outside of the water meter box was made by using a 3/4 compression coupling as is normally used on 3/4 PVC as a repair coupling.
I assume it was done so that the lenght of blue poly inside the water meter box could be adjusted by an inch or two when the water meter was installed 30 years ago. ( The water meter has been removed )
My question is : Is it normal to use the plastic PVC compression coupling to join blue poly, and is it a recommended method....?????
The fitting I saw had been in place for 30 years and there was no evidence of leakage or any problem. I am replacing the blue poly with Sched 40 3/4" PVC as an upgrade.
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