tag monitoring in WinCC explorer

The other day I was troubleshooting a communication problem. The first thing I checked was the tag value in WinCC explorer. It showed quality code 4c fortags that were not updating with the values I new to be in the PLC. I played around with the tags for a while before realizing that it wouldn't update the value until I "called" for the tag value through a picture. In this case, I had an I/O field in my picture. It turned out that the problem was a typo in the tag name in my I/O field.
I would appreciate if someone could explain how the current value and quality code in WinCC explorer is related to the current picture and its contents. Why is the current value in WinCC explorer not independent of the picture such that I can troubleshoot from bottom up (i.e. equipment-radio-PLC-WinCC explorer tag status-graphic on picture)? When the current value is not shown in WinCC explorer simply because a graphic in a picture hasn't "called" for it, is the tag active (HMI querying the PLC for the value) or is does WinCC not request the current value until it is needed? Is there a setting related to this?