In the Release Notes 
vCenter Server 5.1 Update 1a Release Notes
vCenter Server 5.1 Update 1a | 22 May 2013 | Build 1123961
vCenter Server 5.1 Update 1a Installation Package | 22 May 2013| Build 1123966
One is the build number for the installation ISO, the other is the actual software build which explains were your 5 decimal difference goes 