Understanding Tilemap and Pixels per unit

I’m trying to understand better why PPU and Tilemaps work the way they do.

I’m using an 8x8-pixel block for painting the tilemap. When I set the PPU of the sprite to 10, it results in a Tilemap as below, with padding around the tiles.
Screenshot 2023-08-22 231942

Following the answer from here, I set the PPU to 8, and the tiles now fit perfectly.
While this works, I don’t understand why Tilemap works like this. Before using Tilemap, I could set the PPU to 1 or 100, and it didn’t affect any aspect of working with sprites. When using Tilemap, are we always limited to using the sprite’s original dimensions as the PPU?