Hi
A few things that spring to mind.
If insulin is still working, then yes 45mins will give a drop in bg if you test that apart. Difference might not be as high as 8mmol but it depends on insulin profile and how you react to it.
Also, spikes are called that for a reason, they spike high as hell and then drop down. Would also look at ratios for food if you are spiking that high, along with diet maybe?
Meters will give different readings but in my experience, and with my meter, variance is only up to about 0.6 or so at most.
If you are worried, call the manufacturer and get test/ control solution