spectral leakage
A frequency between two bins spreading its energy into both, so neither shows its full size.
Spectral leakage is the spreading of a sinusoid's energy across DFT bins when its frequency is not an integer multiple of f_s/N.
Examples
The lab's fan turns at 23.5 Hz, between bins, so Goertzel on a clean 40 mg sine gives about 25.7 mg at 23 Hz and 25.2 mg at 24 Hz, not 40.
Summing neighbouring bins, or choosing a window length that puts the frequency on a bin, works around spectral leakage.
See also