Levels histogram: gimp vs photoshop

[Carol Spears]

My friend just started playing with photoshop. We had the same
photograph. I loaded it into gimp and took a screenshot of the levels
histogram.

She compared the gimps histogram with adopees and they were very
different.  Unfortunately, she doesn't know how to get a screenshot from
her powerfully expensive program so I am unable to make the same visual
comparison.

It seems to me like the histograms should be the same, regardless of the
program. Is this a wrong expectation?