Three questions: what is X% of Y, X is what percent of Y, and what is the change from X to Y. The third one is where percentages get people into trouble, so the answer gives both figures: the percentage change and the gap in percentage points. Going from 20% to 25% is a 25% increase and a 5 percentage-point rise, and quoting the wrong one of those overstates or understates the story by a factor of five.
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They answer different questions. If a rate moves from 20% to 25%, the gap is 5 percentage points, but the change is 25% — a quarter more than it was. Both are correct and they are not interchangeable, which is why a headline can double the apparent size of a move simply by picking the flattering one. The change calculator prints both.
Because the answer would be undefined rather than large. Nothing can be a percentage of zero, and a change from zero has no percentage — the usual output in that case is Infinity or NaN, which looks like a result and is not. You are told plainly instead.
Only for display, to six decimal places, and trailing zeros are dropped. The arithmetic itself is done at full precision, so a chain of small percentages does not drift.
No. It is arithmetic in the page. There is no reason for a percentage sum to involve a server, and quite a few reasons — salary figures, margins, medical numbers — why it should not.
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