Exactly.
Your other SQL Instances are having memory stolen from them by the OS which is under memory pressure.
My favorite SQL Server memory article of all time says that
SQL Server 2005 does respond to memory pressure both when lock pages in memory is enabled and disabled
It makes no difference whether it's 32 or 64 bit, either
However, the official docs and Slava Oks are at odds with this.
Locking pages in memory is not required on 64-bit operating systems.
(from http://technet.microsoft.com/en-us/library/ms190730%28SQL.90%29.aspx)
I did indicate that on 64 bit platform we recommend to enable SQL Server to use locked pages in memory
(from http://blogs.msdn.com/slavao/archive/2005/11/15/493019.aspx)
I use lock pages in memory on both x64 and ia64.
Craig Outcalt