The variations in level are likely due to temperature variations or slight changes in level. That's why I don't fool around with adding oil until it's at the add mark. That saves overfilling. Plus, many engines prefer the level down from full a bit and will not use oil at a slightly lower level. You keep adding topup, it keeps using it. Some advantages also, such as bolstering the additives a bit. Anway, use some standardized formula (according to the OM), such as always hot (with 10 minutes draindown) or always cold, always parked on the same spot, etc., to get accurate levels every time.
As a tech, I've had to fool around with oil consumption complaints and applied some of what I learned to my own equipment.
To answer the original question, soot doesn't increase volume to any dipstick-measurable extent, though it will increase viscosity as it builds up.