Why has this happened? Shouldn't the old key just get replaced with the updated one?
In OpenPGP (at lest for version 4 keys), the expiry date in not stored in the key packet itself, but in binding signatures (self signatures) on the keys. Changes to the expiry date (and other attributes) are performed by distributing a never version of the binding signature, which then superseeds the old one.
As key servers only do partial verification of signatures (if any), they keep all versions around and have the OpenPGP clients perform verification (and choose the newest valid binding signature and thus expiry date). Furthermore, it offers a kind of history of the key.
This was also discussed some years ago on the GnuPG mailing list, also linking an example showing the duplicated binding signatures that occured because of changing the expiry date:
sub 1024D/2D16624C 2003-05-13
sig sbind FFFD5DA0 2003-05-13 __________ __________ 
sig sbind FFFD5DA0 2003-05-14 __________ 2007-05-13 
sig sbind FFFD5DA0 2011-03-06 __________ 2013-05-13 
OpenPGP clients like GnuPG will usually discard (or hide) the older versions after verifying the binding signatures.
Can I be sure that my published key will expire on the new correct date that I set?
Yes, OpenPGP clients will follow the newest self signature available, and consider the older ones superseded.