Have you noticed that your partners and opponents (not you, of course – you are a very sound bidder) seem to be more likely to have 14 hcp than 17 for a 15-17 notrump? This might not be an illusion, and they might not be wild bidders, either.
Let’s assume that 5-3-3-2, 4-4-3-2 and 4-3-3-3, including 5-card majors, are considered balanced. The 5-3-3-2s represent about 1/3 of the balanced hands. I feel safe in saying a typical expert upgrades the majority of these. Of course, you wouldn’t upgrade QJ Jxx Jxxxx AKQ :-) – but you would upgrade AJT9 KTx Kx KT9x or the like. Let’s assume 1/3 of all balanced hands are upgraded, for now. I don’t think this represents wild overbidding. Subjectively, I’m fairly certain there are people who upgrade more than 1/3 of hands, maybe close to ½.
The important objective statistic is that balanced 14s are about 2.3 times more common than balanced 17s. (Thanks to Richard Pavlicek’s excellent webpage for some of these stats; as he notes, the hcp distribution is different for balanced vs. unbalanced hands.) If your opponent (or partner) upgrades 1/3 of the 14s to 1NT, and 1/3 of the 17s to the 18-19 range, then when he opens a 15-17 1NT you should assume he is about 1.15 times more likely to have 14 than 17! If all the 15s and 16s are treated as 15-17, the overall percentages are that a 15-17 NT will contain
14 hcp: 17% 15: 39% 16:30% 17: 14%
Just hypothetically, if someone upgrades half of all balanced hands, the percentages become:
14 hcp: 24% 15: 37% 16:28% 17: 10%
So 14 would be almost as likely as 16, and much more likely than 17.
6 months ago