How is signal strength (RSS) measurement done on Bluetooth and Zigbee ?
RSSI is not a well-defined term. Every receiver has it's own specific way of estimating it; so the estimation you use for ZigBee might simply be more beneficial to its rssi guesstimate than the method you use for ble.

Also note that Bluetooth (ble not necessarily) is a frequency-hopping-intense standard. Which makes it even harder to cone up with a reasonable rssi formula. Average SNR? Worst SNR in a full hop cycle? BER after decoding? Before?

In conclusion: RSSI is not a scientific measure. It's about as good as the shampoo bottle saying "now 30% shinier hair"

Now, RSS might just be received signal strength in means of physical power - but that is also not a very helpful term by means of digital communication over that link, because what counts is receiver SNR, and that is a whole different story, especially for devices in ISM bands.