my hayward electric spa heater isn't heating as well
Because you're measuring the whole circuit!
I'm sure you'd agree the measurement on the left is correct.



So why does the right side read the same?
The wires connect all those points with an extremely low-resistance path.   So even though you are trying to point to only one resistor with the probes... the wires connect it to the other resistor whether you like it or not.
If you connected the left probe to *anywhere along those wires left of the resistors, you'd get the same thing again.
This is the inherent problem with in-circuit testing of components.  The component must be isolated to get a true reading.