Transverse Decay and Longitudinal Recovery
The decay of Mxy and the regrowth of Mz should be considered separate processes. The net magnetisation vector is the vector sum of these two processes. Although RF pulses rotate net magnetisation, the different amounts of Mxy decay (faster) and Mz recovery (slower) which follow mean that the resultant motion of the net magetisation is more complicated.
These process are intoduced in the Basic Physics section of this site.
Further reading on this topic:
Books: MRI From Picture to Proton p153, 29