I put 0.13 and the system marked me wrong, claiming the correct answer is 0.133. The system is correct. Your answer is wrong. It is important not to throw away information along the way. Sometimes, such reckless rounding can lead to completely erroneous results.
Physical effects are sometimes subtle or small.
The grading system needs to be sufficiently stringent to detect whether or not you have included all of the relevant physics. But It Marked Me Wrong. First, my apologies if indeed there is a mistake. Whenever in doubt, make a printout of the screen and write down the specific things you want me to check. Actual mistakes are pretty rare, however, so check around with other students to see if they are having similar problems.