It is also correct that when our bodies are busy digesting food our BP can be higher as our blood is rushing about doing things, similarly during and after exercise - even walking up the stairs. I know mine fluctuates a lot, even within 5 or 10 minutes. I don't test mine regularly, but when I do, I do 5 or 6 tests in a day, including 2 consecutive ones if it is reading higher than normal. Then I average all the readings over a few days. I also had an episode of what was thought to be sudden drops in BP so had one of those 24 hour monitors attached. The fluctuations on that were noticeable and could be attributed to what I was doing just beforehand. (It turned out not to be BP related, but postural vertigo)
EDIT. Just looked back at my 24 hour stats - it varied from a high of 138/73 to a low of 103/57