This is normal. Even for the L1020, the resolution drops to 0.9MP for smart sequence and burst mode (Microsoft Camera). These are mainly due to hardware limitations where the number of photos queued to be processed necessitates a decrease in resolution for those photos.
A fine elucidation in this process is the news of having 120 fps in Windows Phones in Windows 10 but only in (very) low resolution. More frames squeezed in a short period of time, the lower the resolution gets.