Two IULs can be tied to the same index, carry the same cap, and credit different amounts for an identical year. The crediting method is why. It's the rule for how the carrier turns twelve months of index movement into one number.
Annual point-to-point is the common one and the simplest. The carrier records the index on your policy anniversary, records it again a year later, and credits the percentage change up to the cap. What happened in between is ignored entirely. A crash in August that fully recovers by December shows up as nothing, which is the whole appeal.
Monthly average takes a reading every month and averages the twelve. Then it compares that average to the starting value. It smooths everything. In a year that climbs steadily from January to December, the average sits below the ending value. So the credit comes in lower than point-to-point would have. In a year that spikes early and gives it back, the average can come in higher. Carriers usually pair monthly average with a higher cap or no cap at all, because the averaging does the limiting for them.
Monthly sum, sometimes called monthly point-to-point, caps each month on its own. Then it adds the twelve results together. The upside is capped every month, often at something like 2%, and the downside is not. A single month down 12% subtracts the full 12% from the total. This method looks great in an illustration and behaves badly in a year with one violent month. The capped up months can't make up for a down month that has no cap. The floor still applies to the year overall, so the worst outcome is zero rather than a loss.
Most policies let you allocate across more than one method, and most let you change the allocation at each anniversary. That's a real decision, and the carrier will happily let you skip it forever. Skip it and your allocation was set by whoever filled out the application.
The illustration won't give you a fair comparison. Illustrated rates are built off historical lookbacks, and a method that performed well in the chosen window gets a rate that flatters it. Ask what the actual credited rates were for each method on that carrier's block over the last five to ten years. That data exists, and it beats a projection.
If you're not going to review the allocation every year, annual point-to-point on a broad index is the default that asks the least of you. It's rarely the highest number in the brochure, and it's the one that behaves the way people expect.