Low-pass cutoff frequency definition (-3dB vs. filter design)
I've taken your picture and corrected it showing the real 3 dB point at Fc: -



What you called the "cut-off frequency" is preferably called the lower pass band edge frequency or the lower transition band edge frequency. They are just "markers" that indicate where the real filter response cannot go beyond and are non mathematically related to the cut-off frequency - it is defined by the signal power falling to 50% commonly known as a half-power point.