I have read a Steinberg response elsewhere on the forum that the full .30 installer only updated what was necessary to a previous C8 installation and not the whole enchilada. However, if one had never installed C8 in the first place, it would do a full install. My guess is they didn’t have time (or forgot) to recompile an incremental .30 installer like they did with .40. Hard to say with Steinberg sometimes.
The only real downside to this is the time spent waiting for the larger .30 file to download. As long as the thing does the appropriate updating without totally trashing prefs would be my only concern.
“This download updates an existing Cubase 7.5 installation to the latest version!”.
Since it doesn’t say you need a 7.5.30 installation, just a 7.5 installation, the 7.5.40 should contain ALL 7.5 updates (including all the previous ones, i.e. 7.5.30 too).
The Complete 7.5.30 installation is only for someone who don’t have a 7.5 installation at all.