As far as I remember they are the point releases for
LTS versions - at some point there will be a 10.04.3, which should appear in August I think. 10.10 is not an LTS release
Assuming that you are fully updated when 10.04.1 is released that is what you have, if though you download the iso after 10.04.1 is released then you would not be needing to download/install all the updates that occured between 10.04 and 10.04.1 - and onward it goes.
It would appear it started with 8.04 -
https://lists.ubuntu.com/archives/ub...ne/004318.html