Dragonchilde is correct, when doing pushups, you are not exactly pushing up as you are pivoting your body. That's why the "girl" pushups are so much easier than the regular ones. Not only is it less weight, but the angle is at an easier incline to pivot.
You also will also see a difference if you are lifting "dead" weight, free weights or by machine. Machines are usually easier since they don't make all your little muscles work as hard to balance, so your large muscle can focus just on the weight. Free weights are harder since now more muscles have to come into play to make the lift. And "dead" weight is the hardest since it is not in a nice small medal package with perfect weight distribution. "Dead" weight would be like sacks of potatoes, animals, bags of products, containers of water, etc.
So to give a totally non-scientific example, it would be kinda like this. Let's take a 200 lbs man and this is kinda how it would go.
"girl push ups" 30
Regular push ups 20
Free weights 120lb
Sack of cement 100lb
Just an example, totally not scientific. LOL