Acorn TV format changes for episodes in same season and from month to month

I have Acorn through Xfinity. I like to watch Doc Martin. I've noticed that in the same season some episodes are in HD, some are in letterbox, and some are in wide screen. The wide screen and letterbox distorts the picture. I've watched some episodes more than once and I've notice the format even for a particular episode will change from month to month. Is this something that Xfinity is doing? Would I get all HD if I purchased this service directly from Acorn?