What confuses me about PPLxUL

Hi,

I would like to ask a question about the PPLxUL Split. In the most plans that I find online about the splits there is something that I can’t really make sense of. What I know about muscle growth is that two times frequency is optimal. If you look at some of the plans online, they tend to have for example 3 exercises for back in the pull day, let’s say T-Bar Row, Lat Pulldowns and close-grip row to hit all the areas of the back. But if it comes to the upper day, because it has way more muscle groups to hit, many just remove one of these exercises to keep the volume low. So in many plans that I found some for example remove the close-grip row for the back or others remove the lateral raise for the shoulders - like there’s always one exercise that they just remove. And that confuses me as a beginner because why am I just removing something which I should have to hit twice a week like don’t I have to include even just one set of f.e. the close-grip rows in the upper day as well?