As soon as you open a bag of hops they start to deteriorate, just like other green stuff. If the bag of hops isn't used quickly, which may well happen at smaller breweries, they lose potency.
The brewer should compensate for this by adding more hops as they get further down the bag. A lot don't as they just follow the measurements from a previous recipe recorded in the brew log.
I suspect that you got a "bottom of the bag" brew.