Might be helpful to describe what version you have, what OS you're running it on, and some other info about your system, including video card, disiplay driver version...
I can't reproduce that. It shows up a full pixel size for me (I'm running 13.0.1).
Do you see the same thing if you turn off the grid (View - Show - (uncheck) Grid), and turn on the Pixel Grid?
Seems to me there were some problems seen with the Eyedropper tool introduced in 13.0.2 and 13.1 where it wouldn't sample properly, seeming to find more than one value inside the same pixel. I wonder if that could be related. If it is, I'm sure Adobe knows about it; perhaps the fix is already in the works.
Adobe Photoshop Version: 13.0 (13.0 20120315.r.428 2012/03/15:21:00:00) x64
Operating System: Windows 7 64-bit
Version: 6.1 Service Pack 1
System architecture: Intel CPU Family:6, Model:10, Stepping:9 with MMX, SSE Integer, SSE FP, SSE2, SSE3, SSE4.1, SSE4.2
Physical processor count: 4
Processor speed: 3203 MHz
Built-in memory: 8154 MB
Free memory: 6070 MB
Memory available to Photoshop: 7222 MB
Memory used by Photoshop: 60 %
Image tile size: 128K
Image cache levels: 4
OpenGL Drawing: Enabled.
OpenGL Drawing Mode: Advanced
OpenGL Allow Normal Mode: True.
OpenGL Allow Advanced Mode: True.
OpenGL Allow Old GPUs: Not Detected.
Video Card Vendor: NVIDIA Corporation
Video Card Renderer: GeForce GTX 560 Ti/PCIe/SSE2
The pixel grid works fine and if i turn the grid size up to 4 pixels with 4 subdivisions it works. and magically, when i changed it back to grid size 1 it began to work correctly. Maybe it was a subvision problem... we will never know