Qcache_free_memory not full yet I get alot of Qcache_lowmem_prunes

Question

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     | 
+-------------------------+-----------+