There are a total of 161 perks, but there are supposed to be 357 backers, some are private.
IGG allow creators to hide pledge levels (e.g. early bird once all taken) and also have secret perks (see link in Daves earlier post).
This explains the total amount and backer numbers not matching, but unfortunately means you can't see all the perk quantites like on KS, and it makes it very difficult to check campaigns for dodgy pledge behaviour. If the backer number was closer to your calculated total, then that would have definitely indicated some suspiciously large pledges.
IGG do show a little more info on individual backers and their pledge amounts, except when private option is selected.
KS used to show the full backer list if you kept clicking for more (without amounts), but now they only show a community summary (by country/city) and a small sample.