Usage of the modal SHOULD in clauses of condition

In my grammar it says in clauses of condition should is used to express a supposition that may not be true. For example,

1.If it should rain, they will not come.

2.If he should see me here, he will be annoyed.

I don’t understand what the book means by "a supposition that may not be true". Does it mean it is used similarly to the past subjunctive with if, to express unreality and improbability in the present?