Indeed. But that's the nature of the beast.
In an ideal world, we'd have all this nailed down months in advance, but the problem is, both sides are looking for advantage, and neither side can unilaterally determine what the final result .... unless the UK were to turn round and declare, unilaterally, that we aren't negotiating on anything and come March 29th, it's a no-deal exit. As far as I'm aware, nobody is advocating that.
It's a bit like buying a car, or selling a house, or developing a business contract - terms aren't clear until both sides agree, and so far, both sides haven't. Until a Brexit deal is agreed, and ratified, by all necessary parties, it is simply impossible to be certain what the basis of exit will be.
One example is May agreeing the deal, and the Commons nixing it. Another would be her reaching another desl with the EU, the Commons ratifying it .... and then the EU parliament, or a member state, nixing it. Or, an art.50 extension being agreed with the Commission, then a member state refusing ti ratify it.
The nature of the exit cannot be certain, until the deal is done and ratified by all parties, or until the 2-yr clock expires with no deal agreed.