How can flood inundation vary at the same river stage?
Hydrology major here, for whatever that's worth :) Some possibilities from my readings/internships:

First I'd check the gage you're using for stage--if it's moved in the last 8 years, the stage may need to be correlated to what it would be at the former site. If it hasn't, it's more likely something changed in your stretch of channel. 

If the channel has deepened at your location and not at wherever the gage is located, then you'll see less flooding. If the gage is upstream of your stretch of stream, I'd be suspicious of any new wetland areas that could accommodate water; if it's downstream, of any dam removal or other loss of channel restriction that could be allowing water to get past your site more quickly. Downstream changes in the channel could also affect your site! And if it's fairly urban, maybe the city has figured out ways to divert runoff farther downstream?