|Depending on the tester, the tube usually serves simply as a rectifier. In more elaborate testers various voltages may be applied to the grids. In these testers the tube may or may not be fed pure DC. However, in a majority of testers (emissions), the tube is fed raw AC. The meter reading you see is indication of its passage of current in one direction. If the tube was shorted, it'd feed the meter AC, and would most likely cause it to read low (in my experience) due to the AC swinging the meter needle back and forth very quickly instead of sending it to the right like it's supposed to.
In my experience with my tester, it is normal for some tubes to test extremely well. I have found that they're usually made by Sylvania, but that's a different story. At any rate, meter pegging is an indication of very good cathode emissions. Also, a bit has to do with what the tester designer considered a median "GOOD" reading. For some of my tubes, namely 6K6, I swear that the settings are a bit low, because no matter what 6K6 I plug in, they all test on the low end of GOOD or worse, but never higher. Could be that all of my 6K6s are weak, but I sort of doubt that. I wonder what EICO was thinking, because a 6K6 and 41 will have different meter SHUNT settings even though they are both the same tube electrically.
Anyway, a tube is most likely very good if it pegs the meter. If all tubes of that number read extremely high, it may be that the tester designer picked rather liberal settings.