The reason to call of a strike is actually to be able to pass a _bad_ deal. You see, the workers are not very likely to accept a bad deal if they've just come out on strike - even if their leadership has made excessive concessions in the negotiations and is putting it up to a vote. If, however, the leadership has "taken the wind out of the sails" of the struggle, the demobilized workers have the gotten the message that their struggle is over and there's nothing to be done, and are much more likely to accept it. This is usually accompanied by union leadership publishing glowing descriptions of all of the wonderful gains, usually in vague and confusing terms; never listing the set of original goals; and not mentioning what concessions were made.

An uncorrupted union would never calls off a strike because of an employer proposition: If the proposition/proposed contract is offered enough time before the strike was decided, then it is brought up for discussion - with the negotiations team presenting both the benefits and the detriments: What they got and what they conceded; and the result of the discussion decides the fate of the planned strike. But when an offer is made just before a strike, naturally - the strike proceeds, and the offer is discussed in parallel with the strike. If the offer is accepted, the strike ends up being a short one; and if it is rejected - the struggle continues with the "wind in their sails".