Data retention voltage could be explained as the lowest possible power supply voltage at which the data can be retained inside the SRAM. One thing to remember is that the chip is deselected at this point of time.
But when power pin be given a voltage less than the minimum retention voltage, the contents of the memory may not be erased and when a read is initialed the value of certain location may be same as before. When a memory device powers up, the contents of the memory cannot be determined as the cells of the memory powers up in a random way ( 0 or 1).
E.g. For a particular address location, if we write a known data and power down and power up the chip immediatly, if we read the particular address location the same data could be read thereby giving an impression that data is retained irrespective of power down.
If it is a security related application where contents of SRAM need to be erased on detection of a tamper event, it is recommended to write the critical locations with junk data on detection of tamper event.