The Maharal says that the sponge, funnel, sieve, sifter, are not about memory because that was explained in an earlier mishnah - those who forget more or less easily... It's not about what they're generally able to remember or not remember. It's about how they react to the differing opinions of the chachamim. That's why this mishnah speaks of those who sit in front of chachamim (as opposed to students). Each of these have their own way with all the variant opinions of the rabbis. One can't differentiate the differences, takes them all in equally. The next is allergic to differences of opinion, he can't handle it, take it in, keep it in. The next can't tell what is clear and what is off, what is the halachah and what is not. He thinks he gets it, but he switches the two, had poor judgment and chooses the wrong parts of what's been said. . The Maharal says that this applies to many people - they don't discern what makes sense and what doesn't beca