Why so different versions for Bind9 exist?

[iman]

Hello ISC Admins
I`m new worker on bind.
I want to know what are the main differences between types of Bind9
versions?
why so different versions exist for bind 9
ie 9.3.x-9.4.x-9.5.x? and ISC updates each of them separately?
why isc doesnt relaese one version for example Bind 9.1, 9.2,
9.3 ,...and increase one number after bind for newer version?
and the main question is, what are the important options that we must
consider for selecting which version of bind9 is useful for us?
bind 9.3.x
bind 9.4.x
bind 9.5.x
any idea would be appreciated
Regards