Some cells depend on others. Some dont. Change one cell and it usually has an effect on others. Some changes are instantaneous, some takes longer. Some take long enough that you can't figure what caused this effect.
The only difference vs excel is here is we only get to see the cells, and their values, not the formulae behind it. And we are systematically trying to reverse engineer the formulae from the changes we see. We find some patterns, think its a formula, only to later find a contradiction, and that we don't know enough of the formulae yet. In some cases, the contradictions haven't been found just yet.
Theory after theory, change after change.