Information can be found here: https://www.minecraftforum.net/forums/minecraft-java-edition/redstone-discussion-and/2933632-1-13-2-bug-items-in-hoppers-giving-off-different

basically, some items when placed in a hopper are giving off different redstone power levels when a comparator is facing out of the hopper. 

A minecart by itself in a hopper seems to be giving me a power of 3. This is abnormal. 

A sign and 19 other blocks is giving me a power of 2. This is abnormal. 

cobblestone and 21 other blocks is giving me a power of 1. This is normal. 

There are are more items giving off something similar. Some are normal, some are abnormal, some are very abnormal.