I observe this behavior, too.
I have no explanation to offer and no help.
System Info:
Lightroom version: 4.3 [865747]
Operating system: Windows 7 Business Edition
Version: 6.1 [7601]
Application architecture: x64
System architecture: x64
Logical processor count: 4
Processor speed: 3.0 GHz
Built-in memory: 12285.5 MB
Real memory available to Lightroom: 12285.5 MB
Virtual memory used by Lightroom: 685.2 MB
Memory cache size: 0.0 MB
Maximum thread count used by Camera Raw: 4
System DPI setting: 96 DPI
Desktop composition enabled: No
Displays: 1) 1920x1200
ACR Cache: 75 GB