I don't think you are giving enough credit to the committee members. Suppose you the student is named Joe, your recommender is Prof. Simon and the committee member is Prof. Addy.
First, recall that both Addy and Simon are some smart folk. Second, Prof. Addy has Joe's entire record laid out in front of him.
So, imagine Joe says that he has done 2 years of research with Prof. Simon. Prof. Addy would probably notice that Joe was a full time student during this same period. Opss, obviously Joe was not doing full time research! This scenario takes care of most of the Joe's.
Now suppose that Jeremy took a year off after graduation. Prof. Simon was his supervisor during this period when Jeremy was doing full time research and Prof. Addy is once again on the committee this year. He sees that Jeremy does not have any other activities during this time. He sees that Jeremy has listed his research in the Employment History section since Jeremy would have to be getting paid (or starving...). In addition, Prof. Simon's LOR would reflect how closely him and the student were working and trust me 10 hrs/wk is nothing compared to 40 hrs/wk. Let's not forget that if Jeremy is spending all this time doing research he will be sure to make it absolutely clear in his statement of purpose. Surely, Prof. Addy has seen so many statements he must have a pretty good idea what the student meant to say and even things they did not mean to say but accidentally did so indirectly.
Now you might have some additional points:
1) Prof. Addy wouldn't be looking as hard. I beg to differ. These professors realize that the students they admit will work with them for the next 6 years; the students will therefore have direct influence on the quality of the research done in the department and their own groups reputation. I bet you they are putting forth their best judgement and we better trust that that is good because once accepted these professors and their judgement will be the once influencing where we end up in our careers.
2) You might say that Prof. Simon might write an unfairly good letter. Once again I beg to differ. Prof. Simon submits not only a letter of recommendation for you. He submits a whole package for you, which includes a letter, his reputation and a statement of the quality of his judgement. Now if this year Prof. Simon submitted a letter that was saying the student is extraordinary, but the student turned out to not do well after acceptance. Then the next year when Prof. Addy sees a letter written by Prof. Simon he would be much more suspicious of the students abilities. Now, do you think Prof. Simon would have risked his reputation by writing a magnificent letter?
Anyhow, thanks for reading, this got a bit long