In the obvious way that when A and B are similar enough (as font variations are) a delta between A and B is substantially smaller than bundling A AND B.

Are they similar? I would expect that between a bold glyph and a normal weight glyph, exactly zero points would be the same.

In the abstract, no, but in 99% of the cases where it's used in computing, e.g. it software updates, rsync, etc, it does, and that's exactly why it's used.

You're comparing vector deltas (which are presumably floating point) to deltas between binaries, where you can take a line-by-line or byte-by-byte diff. This is apples and oranges.