I checked the datasheet. The Xi 2550 has a quite weak battery (only 2200mAh).
I think the 2 hours that are stated there, are nearly impossible without major modifications.
Mine has 4800mAh and I can only reach the 4 hours stated by FSC with a lot of tricks.
I think for you 90 minutes is a realistic runtime with a new battery.
So depending on the age and some other influences 70 minutes seem to be ok.
How old is your notebook?
And how did you treat the battery? Has it always been inserted or do you remove it, when you work with the power pack? At which capacity do you you normally start to recharge it?