Switching between age and grade norms switches the makeup of the group you are comparing your examinee to. Standard scores and percentile ranks provide you with an indication of where your examinee stands/falls in comparison to their peers.
Example: You are testing a 9 year old in the 4th grade.
When using age based norms for a 9 year old, you are comparing them to same-aged peers (other 9 year old's) in the norming group.
In turn, when you use grade based norms for a fourth grader, you are comparing them to same-grade peers (other fourth graders) in the norming group.
Whenever you change the norming/comparison group, there is the reasonable expectation that your standard scores/percentile ranks will also change. This is because where an examinee falls when compared to same-age peers may not be the same as where they fall compared to those in their same-grade.
Analogy:
If you were trying to compare your height to your coworkers, you might be in the middle (3 coworkers taller than you and 3 shorter than you).
But if you went to the grocery store and tried to compare your height there, you might be a different ranking (2 other shoppers taller than you and 6 shorter than you).
In this case, your ranking, changes depending on who you're being compared to.
In a similar vein, your examinee's ranking (which is what a Standard Score/Percentile Rank gives us) changes depending on who you're comparing the examinee to.
