The lower the frequency you are using the more powerful the return signal will be because the longer wavelength of the lower frequency will transmitt thru the water more efficiently than the higher frequency ........ So I would consider it normal to need to increase the sensitivity with the higher frequency.
Of course this all depends on what the programming in the unit changes when the frequency is changed. It would be possible to automatically compensate the sensitivity range when changing the frequency ... and this could vary from one model to another, one software update level to another, and even the bottom and water composition from one lake to another.
Bottom line is that there are a lot of variables that affect what sensitivity setting is required and as long as you are able to adjust the sensitivity to work well with each frequency, I wouldn't be concerned about it.
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