The carbon we release in driving to the beach is actual but the carbon we save is more notional. If you are subsidizing a company that gives out free energy efficient light globes - how much does that actually save is an actuarial exercise in estimation. One is happening now but the savings are projected to happen in the future.
But what if you are paying somebody not to do something in the future, like not cut down that tree. Now that's really notional. Farmers may be asking for credits for trees that they could have cut down but don't actually have any plans to.
To the cynical offsets could be seen to just be subsidizing normal activities that would have occurred in the normal course of events - and therefore increasing profits.
Offets are also used to excuse the unsustainable growth in industries say like travel that emit a lot of carbon.
There also the issue of discount rates for carbon savings which I'll deal with in another blog.