I assume you disconnected all the wires to the elements before measuring? Or the common neutral will provide misleading readings, some elements my also be linked together during certain selector switch positions as well. Unusual for so many to be low unless the oven hasn't been used in a while.
Might be worth running each element for 20 minutes or so and re test again. If they are still low, replacing the elements is likely the only solution.
If there is damp in the elements mineral insulation, the reading will often get even lower when it's hot, until the moisture has been driven out.