Hi,

1. MapRoulette isn't always right. 
2. As you can see on the aerial photographs there are 90 degree angles in the front side of the houses. You made it 1 smooth line. That is not correct.
3. Dutch buildings are imported from the Dutch official surveyor's building registry (Kadaster). They are (almost) always right, but sometimes strange rounding errors appear. These small mistakes will reappear when a new import is done.
4. The narrow part of the wall that sticks out is visible at https://maps.app.goo.gl/SZRr98tX14AzA9nr6

I'll leave it up to you if and how you change the 'error', but don't 'cut corners'.