Wireless N not faster than my wireless g

help please, have a TP-Link G router, was getting 736kb download, have set up a TP link N (300) but am only getting 560kb download, (my standard BT home hub N was also not as good giving me 640kb download....
with Wired conection results were....
TP-Link G - 752kb
BT Homehub N - 648kb
TP-Link N - 590kb

laptop - Dell XPS, with N wireless
Windows Vista, sp2.
used www.broadbandspeedchecker.co.uk to check all the speeds.

Any ideas as to why the TP-Link G is so much faster than both the BT Homehub N or the TP-Link N ?