interpret wavelet scalogram
Wavelets are ideal for localized events. The Fourier Transform represents a function as a sum of sines and cosines, neither of which are localized. The spectrogram does keep some time information, at the expense of frequency resolution

In your case, the signal is not localized at all.  The spectrogram smears your 15 Hz band over several Hz, as it captures some time information, and the scalogram flat out fails.