Avis can be stingy with upgrades at some locations. It's obviously not due to your profile if it says it on the rental card. Probably a function of time (do you show up on Tuesday/Wednesdays when they are low on vehicles) or the station (is it the same location or different ones)?
And are you sure you're getting Intermediates and not a full/other? Many cars that I'd consider intermediate (Camry, etc) are full size by Avis standards.