I'm sure the report says her arms and legs weren't in the hard suitcase type item her body was found in so maybe it's possible that they could see bruising on the lower body either because
1) the exposed limbs weren't as decomposed as the part that was in the bag, then wrapped in the sheet or
2) the part that was in the sheet was more protected and her arms and legs suffered more decomposition.
If they haven't been able to give a cause of death as yet then that suggests the upper part of her body was more decomposed though.
Not sure she was in a "hard suitcase type item". That was an early report in the Daily Mail. Later reports in most papers state she was wrapped in a black sheet and put in a black bag. Some reports seem to have interpreted the black bag as being a black plastic rubbish sack.
The body of 12-year-old Tia Sharp was found wrapped in a black bedsheet in a black bag in the loft of her grandmother's house, it has emerged.
http://www.guardian.co.uk/uk/2012/aug/13/tia-sharp-body-found-bedsheet-loft?newsfeed=true