Why By convention an S meter is calibrated so that each S unit represents a 6 dB change in received signal strength, which is a factor of about 2 in voltage or 4 in power. The reference point is S9, defined on HF as 50 microvolts at the receiver's 50 ohm input; above S9 the scale is marked in dB over S9 rather than S units. So a report going from S5 to S7 means the signal came up roughly 12 dB.
Watch out The 12 dB choice is what two S units represent, not one, and the larger figures do not correspond to any standard S meter step.
One S unit = 6 dB = double the voltage. S9 = 50 microvolts.