This is the case when diffusion in the interior is possible (like in the liquids) or when the initial small crystals are allowed to recrystallize. Kinetic effects (like speed of crystallization and presence of mixing) play a role.
This situation arises when molecular movement within the interior is possible (e.g., in liquids), or when the initial tiny crystals are allowed to reform into larger crystals. The rates of crystallization and the effects of mixing also play a role.