Apply a little critical thinking to the above,

aim to move towards the 2% guideline within a decade with a view to
meeting their NATO Capability Targets and filling NATO's capability
shortfalls.

Through the NDPP, NATO identifies the capabilities that it requires, and promotes their development and acquisition by Allies.
The NDPP provides a framework within which Allies harmonise their national defence plans with those of NATO, without compromising their national sovereignty.
The NDPP apportions requirements to each Ally as Capability Targets on the basis of a fair share of the overall requirement, facilitates their implementation and regularly assesses progress.

So for critical thinking analysis, the questions would be-

Has your hypothetical ever happened? No. What are the probabilities it could happen?

What are the most common reasons for a shortfall?
Are there any extenuating circumstances? Undue hardships?

What are the NDPP assessments regarding these shortfalls?

The pledge isn't to make one minor incremental move in 10 years, the pledge is to implement and increase in investment and over 10 years reach the 2% of GDP level. I would think the biggest reason is not wanting to deal with the budget pain.