Actually, they're in two different sorts of units. The signal and noise measurements are in decibel milliwatts and the SNR is in plain decibels. dB is a meausement scale that's only useful for comparing one value to another. In the case of the signal and noise measurements we choose 1mW to be 0 dB and measure from there (hence, these measurements are usually a negative number). In the case of the SNR we are comparing two values directly and it doesn't matter what value is used as a zero for each measurement as long as it was the same. The difference gets lost in the subtraction.
Signal = -38dBm = 1mW - 38dB
Noise = -96 dBm = 1mW - 96dB
SNR = 38 dBm - -96dBm = (1mW - 38dB) - (1mW - 96dB) = (1mW - 1mW ) + (96dB - 38dB) = 58 dB
But nobody but a hardcore radio geek is going to call you on it if you just refer to everything as dB.