Qcache_free_memory not full yet I get a lot of Qcache_lowmem_prunes

I just started dabbling with the query cache for our CMS. Can anyone tell me (or at least give a good guess) why I get **a lot** of Qcache_lowmem_prunes when more than half of Qcache_free_memory is free? query_cache_size=512M query_cache_limit=1M This is how it looks after about 12 hours show status like '%qcach%'; +-------------------------+-----------+ | Variable_name | Value | +-------------------------+-----------+ | Qcache_free_blocks | 10338 | | Qcache_free_memory | 297348320 | | Qcache_hits | 10254104 | | Qcache_inserts | 6072945 | | Qcache_lowmem_prunes | 725279 | | Qcache_not_cached | 2237603 | | Qcache_queries_in_cache | 48119 | | Qcache_total_blocks | 111346 | +-------------------------+-----------+ This is how it looked after flush query cache; show status like '%qcach%'; +-------------------------+-----------+ | Variable_name | Value | +-------------------------+-----------+ | Qcache_free_blocks | 1 | | Qcache_free_memory | 443559256 | | Qcache_hits | 10307015 | | Qcache_inserts | 6115890 | | Qcache_lowmem_prunes | 725279 | | Qcache_not_cached | 2249405 | | Qcache_queries_in_cache | 26455 | | Qcache_total_blocks | 54490 | +-------------------------+-----------+