A question similar to that one came up in a diabetes management education class I was taking. My understanding is that meters on the market are about 90% accurate in there readings, so your meter is most likely fine. Home glucose checking is designed to only give a ballpark snapshot of what our blood glucose is at that moment of time. Blood flow through the body is dynamic and the chemistry, including glucose, at any given point will very from one moment to the next. That said unless you took a blood sample and tested it many times (not taking a new sample for each test, thus a different blood chemistry) the variance in readings you recorded are normal. Most health educators that I've had contact with, in person or read articles by, suggest taking only one reading at a time since monitoring is designed to help us to see trends that raise or lower our glucose (using some form of written tracking is beneficial that way we can look back over a week or more to see these trends) so that we can make changes in our lifestyle to better control our glucose.
Just a note on taking a reading after a meal unless you took a reading before the meal you won't be able to tell much form the reading. An example if my after meal reading is 200mg/dl, what does that mean about the meal that I ate? Was it not diabetic friendly? Without a before reading that would be impossible to tell. If my before reading was 180mg/dl then the meal was OK. If it was 100mg/dl that would indicate that the meal should be avoided or adjusted to be more diabetic friendly.
Very unusual. We can't see where you test or how you store the meter. Environmental factors can affect a meters performance. Being close to radio waves from a wireless phone [cell or land based], computer or microwave running for example may cause interference. I always have on hand two meters, one is my backup.