I wouldn't call it weird. I don't know why they didn't have credits on every show in the first place.
And the time-frame thing is just my own speculation. It doesn't really add up that certain segments will be in one episode, but then cut from another one. Maybe trying to reduce the amount of repeated material for the sake of residuals?
In the past, somebody on this forum said that the reason they only had credits once a week was because they felt the time allotted for end credits could be better used to teach more lessons, but Union rules mandated that they had to have credits at least once a week.