One thing to keep in mind is that dB (decibels) is a *relative* measure of a sound level, sort of like the word "percent" or "times" (a sound is X percent louder than another sound, or Y times louder, etc). A 6dB difference in level is roughly a doubling of amplitude.
Many meters in DAW products use "dB" when they mean "dBFS" because "dB" is just shorter and nobody wants to say "dBFS" out loud. dBFS stands for "decibels relative to full scale" which means the level relative to the digital maximum, the highest amplitude representable in DAW signals. When a sound is -6dBFS, it's about one half of the full amplitude it could be without clipping, or, 0.5 where 1.0 is the typical maximum.
So, the dB scales you're seeing here may be using different reference points for the dB measurement.
I'm not 100% sure they are, because there are other reasons they might differ such as different pre-processing of the signal before analysis, different parameters of the FFT analysis itself (window functions, block size), averaging or other modifications to the FFT results, psychoacoustic scaling of the frequency response (such as applying an artificial gain to higher frequencies), or even just different decay/falloff rates on the graph.
Oh right and as others have mentioned there is also peak vs RMS.