What causes this irregularity? Shouldn't we memorize all pages of the piece equally?
You see, it depends on the complexity of the episode, especially in terms of texture and musical language. If an episode has a homophonic texture (melody plus accompaniment), then obviously our ears pick out the melody much easier. The accompaniment usually consists of patterns and we notice them right away. This also makes it easier to memorize.
But if the episode is polyphonic with several independent melodic lines interacting at the same time, then obviously it's much more difficult to memorize it. The same can be said when the musical language is highly modern and seemingly doesn't make sense.
In such case I recommend taking the episode apart and memorizing it systematically, either part by part (separately) or measure by measure (the entire texture).
If you practice each step slowly and at least 3 times in a row correctly, then chances are you won't forget the complex episodes so easily.
Whatever you do, make sure you analyse the piece first so that even complex episodes make sense.