That's pretty standard, and there's also a budget to contend with so in on some episodes/scenes they splurge and in others they let go a bit.
I'd like to think that all shows have unlimited budgets, but sadly that's not reality... as much as we'd like for quality to be consitent (art/animation wise), it simply isn't feasible.
Of course, sometimes the low budget portions are unexplainably bad.
A term I've seen used around often is "sakuga", like scenes in which more budget to increase the quality of those particular scenes, and such or something along those lines.
But yeah, all can be explained by budget constraints, though of course some is laziness at times. One can be more understanding of these issues, but doesn't make them any less annoying when encountered.
That said, though I can appreciate great animation, poor animation often doesn't bother me. It's not a make or break issue for me unless this something incredibly substandard.