Without knowing the internal design of your exact model of metronome it's impossible to say. All metronomes I've come in contact with do not display this behavior.
I will say, depending on how many corners were cut in order to produce the metronome as cheaply as possible, it's plausible that your metronome does do this. However, whoever designed the metronome likely did not intentionally give it that feature. A Duty Cycle is a waveform that many undergraduate engineers will encounter at least once. Simple and cheap to create. If you consider a metronome to be just an Impulse happening at a certain frequency, then a duty cycle with a very short period models that relatively well. Of course it has the side effect of shortening the duration period as frequency increases, which would cause your little metronome phenomenon.
Above is a gross simplification of a potential metronome design. Do I think that is how your metronome works? No. I think it's just some aural trick making your ears think the click is shorter, and that your metronome is just like all the others that we use.
a. Some metronomes have an option to include them in the beats. Most simple ones do not. It's good practice to just subdivide the beat yourself, in your head.