It has to be dissolved, absorbed and in place in the Delta Cells of the pancreas at the time when food is first swallowed. This is the timing of the first insulin response. Within 10 minutes of eating it is all over. This is the kick in the pants that triggers insulin release.
Edit to add. Looking at the question of the readings, then errors in measuring could contribute about 2 mmol/l due to meter uncertainty. So a glucose rise of 2 mmol/l from the cracker is plausible. In terms of LowCarb, most cracker run at around 75% carb (eg Ryvita, Ritz) so are not helping an LC and must be portion limited.