There is a very small variation between levels. By the blue level, it is very hard to detect that words are rationed. Children can be successful readers, so they want to read these stories. Wide word spacing makes visual distinction between words easy for readers. This is especially important for struggling, new readers.
Many characters reappear thruoght the PM books.
Children relate to these familiar characters and look forward to reading about them. Fiction stories with science topics are scientifically accurate. Children enjoy fiction stories that also teach science concepts. The PM levels build on each other and high frequency words are revisited frequently. Teaching high frequency words in context is important.