What does the 902-927Mhz in UHF RFID means?
...How is it possible?

Easy, it's not that tag which detemines the frequency but the reader.

The tag works between 840 - 960 MHz. Also that tag is just some antennas and a chip. The tag has no battery, it gets its power from the reader via its antennas. 

The tag also does not have a way of generating a precise frequency. For that it would need a crystal which would make the tag more expensive. And there's no need for that. The combination chip + antennas is just made such that it can work between 840 - 960 MHz.

Now the reader is more complex, it also needs antennas to communicate with the tag. It needs a power source like a battery, adapter or USB connection. It will also have a crystal for generating a precise clock. This allows you to set it to a certain frequency.

As long as that frequency is within the suitable range for the tag, it can be read.

When the tag receives the signal from the reader, that signal is used to power the tag. To transfer the data some clock is needed, it is possible that the tag divides down the frequency it receives from the reader to a much lower frequency and it might use that as a clock to transfer the data. It is also possible that the tag uses its own (less accurate) internal clock and that the reader simply derives the data clock from that signal.