Even if you'd tested the same sample 3 times in the SAME meter, the readings could have varied just as much or even more. Though some meters are also known to be, on average, higher or lower than others. To make things even more confusing, some meters differ more from others when the true value is low or high.
A modern meter should be accurate to 15%. When small differences matter, that's a lot of inaccuracy. The only way to be absolutely sure of what your true level is, is to take several readings and average.
E.g. If your true level was 7.2, you could easily get an 8.3 on a modern meter.
Things get even worse when measuring before-and-after-food. If your before reading is much higher than 'truth', and your after reading is much lower than 'truth', you could think you've eaten a safe food, but it's actually not safe at all.