Does anyone have an explanation?
Yes, this is what will happen when a squarewave input is processed by an RC differentiator: -
The output vertical edges are constrained to an amplitude of 3.3 volts but, once an edge has occurred, \$V_{OUT}\$ fairly rapidly discharges to 0 volts due to the charging of the capacitor via the resistor. The next edge (of the opposite polarity) drives \$V_{OUT}\$ in the opposite direction.
The above picture is 10 kHz but, as frequency rises the p-p will tend to eventually become the same as the input waveform. At 100 kHz \$V_{OUT}\$ is about 5 volts p-p: -

