This is from memory and might be a bit inaccurate, but:
At the end of Portal 2, you’re trying to corrupt Wheatley with those three corrupt cores. When you attach the Space Core, the corruption hops up to 50%, but the other two cores increase it both by 25%.
This could either mean that the Space Core was more corrupt than the other two. This makes sense but seems a tad arbitrary. The other interpretation is that Wheatley was already partially corrupt. Perhaps a slow corruption from the chassis itself or a result of him being turned to crushed wheat when GLaDOS woke up.
On another note, GLaDOS is 80% corrupt. You knock the 4 cores off of her in the first game — so if each core is 20% and GLaDOS herself is 20% that might explain it.
This probably doesnt make much sense at the moment. Sorry I’m tired.