Sample size (12 reeds per chunk) was far too small to give accurate data given the random ticks can be applied to any block in the chunk. A more accurate measure would be to fill a 16x16x16 area with cane farm in each chunk and let it run for an hour. Each 16x16x16 chunk gets the random tick 3 times per game tick, so you would need to make sure the 16x16x16 cuboid was in the same y-chunk for accurate reporting.
I guess u can make a bigger afk pool that goes throught the chunks (wont work w/ too many) but otherwise that might work worth giving a try atleast Good Luck