Because of how ICE is optimized, it won’t bother setting data if you never use that data (see Beware of ICE Optimizations here). For example, suppose I have a point cloud where I set a per-point scalar value, but I never use that scalar data (because I just want to cache that value and use it later).
That scalar attribute is never used, so it is optimized away (it makes sense: you’re not using it, so why keep it?). You won’t see that attribute in the Cache Manager, and the Cache on File node won’t write it out (even if you add that attribute to the list).
And last but not least, if you have emTools from Mootzoid, you can use the Force Value Evaluation node, which uses a Show Values on a value that is never set.
hat tips to Stefano, Mathieu, and Vincent on the SItoA list