What causes a 'read' on a block calculation in the logs?

Summary:

What would constitute a Block Sparse Calculation Read in the calculation logs?

Content (please ensure you mask any confidential information):

I am working on some different methods of calculation and I'd like some clarity on something I'm seeing in the logs regarding "Blocks Sparse Calculations: Reads and Writes". I'm trying to minimize unnecessary reads over blocks wherever possible by scoping my script accordingly. However, one nuance I'm noticing in my calculation is that I can't make complete sense of how the calculation block reads are actually computed, and I think I may be misunderstanding something here.

I have a single block in a very small cube where Year and Product are Sparse and Account and Period are Dense. See example Smartview retrieve: