BooleanQuery

[WATHELET Thomas]

Why I retrive hits with this query : 

+doccontent:avian +doctype:AM +docdate:[20050331220000000 TO
20060620220000000]

and not with this one 

+doccontent:avian influenza +doctype:AM +docdate:[20050331220000000 TO
20060620220000000]

????