Could be any number of reasons but 2.9 is within mint tolerance for a copper cent. Would help if you posted a full pic of both sides of the coin though.
Your scale only goes to tenths is one reason. Maybe it's 2.94 or 2.95. Maybe it's calibrated correctly. Did you weigh other cents as a control measure? 37 years of wear and circulation can easily grind off a tenth or more. Maybe it's a slightly thinner rolled planchet.
This coin is within tolerance of both a copper cent and a copper plated zinc cent? can someone post what the tolerances are? Its ~.2 grams below copper and ~.4 grams above zinc. Which puts it closer to a copper cent. The anomaly on the reverse on the 2nd T of STATES make me lean towards a copper plated cent. There looks to be zinc showing through.
Zlincon: 2.5 +/- 0.13 - https://www.law.cornell.edu/uscode/text/31/5113 How sure are you of the calibration of the scale? There are two things in play, the accuracy of the scale and the accuracy of the display... Most scales are only +/- 10% accurate (esp. the cheap ones), with that, 2.9 could be 2.61 to 3.19 A display of 2.9 could be from 2.85 to 2.94... You can build confidence in the scale by checking a few dozen coins and making sure they are showing the expected value.
I've checked over 100 coins and they read 3.1 or 2.5. I did this coin more then 10 times and it reads 2.9 everytime..
If it's a zinc coin, then it could be a thicker rolled planchet. If it's a copper coin, then what I already mentioned. They made both types of coins in 1982, it is a transitional year as they switched from copper to zinc. My guess is that it is a copper coin, since they made a ton of large date copper Philly 1982 coins, and the very slight difference in weight adds no extra value.
It's zinc and in tolerance. It might be a bit off if the planchet was rolled thin or thick but it's normal.
If it's zinc it is nowhere close to tolerance. By appearance I would say it is copper. And because the weight is closer to 3g, also probably copper. And just off the low end of the copper tolerance. Circulation and wear most likely lost some of the weight. It's a penny.