darker in aperture

I take photos using an iPhone 4s, sometimes using HDR sometimes not.

I upload them to iPhoto and they appeaar the same as on the phone.

When I upload them to Aperture, they appear darker.

The photos have been taken in a range of ligh settings, some dusk and some on an autumnal UK day.

I can make them light using the modification settings, but I would be interested as to does this occur to other users and what it the cause.

As ever, any relply appreciated and I thank you in advance

Phil