Windows Experience Index adversely affected by "Enable vertical synchronization"

I am running Windows 7 Ultimate in a VM on the following Macbook Pro:

Model Name: MacBook Pro
Model Identifier: MacBookPro2,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.33 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 3 GB
Bus Speed: 667 MHz
Boot ROM Version: MBP21.00A5.B08
SMC Version (system): 1.14f5

My VM is configured as follows:
1 CPU, 1024MB memory, 256MB graphics memory, Enable 3D acceleration is on, Enable vertical synchronization is on.

Prior to the 9308 update, this VM had a Windows Experience Index score of 2.9.
After the update, the score dropped to 2.0.

Thinking that enabling vertical synchronization might be the issue, I turned this off and then, I reran the assessment.

With vertical synchronization turned off, the Windows Experience Index went up to 3.9.