Most level meter scales are logarithmic. In the days of analogue meters it made a compact and easily read instrument as it directly indicated the power of the signal against the chosen reference value. The arrangement put the end stop of the meter virtually at infinity, and saved bending the needle round it when levels changed unexpectedly. Maybe there's no need to do this with digital meters, as there's no delicate needle to bend, but the convention has stuck.
The base unit for comparing the power levels is the Bel, which is equal to the difference of the logarithms of the values. in this case, log10 10,000 is 4, and log10100,000 is 5. The difference is 1 Bel relative to 1 Watt.
The Bel is a bit of a cumbersome unit to juggle in mental arithmetic, most of the time the significant digits are to the right of the decimal point, so the decibel (1/10 of a Bel) is commonly used, usually bringing the most significant digit to the left of the decimal point.
So the above tenfold increase is commonly expressed as 10dBW.
In the past we were more familiar with logarithmic scales as they were commonly used on slide rules.
So to find the increase in power from your meter reading, you need to divide the levels by ten to find the values in Bels, then find the antilog of the values to determine the power levels. In this case:-
50dBW= 5 Belwatt, and
40dBW = 4 Belwatt.
Log^-1 4= 10,000 Watts
Log^-1 5= 100,000 Watts
