Launcher redownloads itself periodically after big updates - why?

I live in the country, with limited bandwidth, and every time a big patch comes out for the launcher the game seems to just redownload itself from scratch. This has caused me to delete the game entirely as I simply cannot afford to download 20+ Gbs every few weeks or months as that's a massive chunk of my monthly allowance.

Does anyone know why this occurs?

The launcher downloads the new patch and then, once it's finished, proceeds to just attempt to seemingly download the entire game from scratch.

For example, today I started the download for a 1.2 Gb patch and assumed it would unpack and install afterwards but instead it just went to download an additional 18 Gbs. Now every time I open the launcher it just immediately starts downloading 18+ Gbs and won't allow me to launch anything. This has happened to me multiple times over the last year or so plus.

What gives?