May I suggest that anyone who is told at check in that they have excess hold (or hand) baggage weight requests that their bags are re-weighed on a DIFFERENT set of scales before accepting the charges to confirm the accuracy of the displayed weight. Just the way that a large piece of baggage is wedged or otherwise on the baggage belt MAY affect the perceived mass of a bag and there is always the issue of confidence in the accuracy of the measuring device.
The small spring-operated hand scales give an "approximate" idea of the weight of a bag but cannot be relied on, especially over time, to be 100% accurate The airport baggage scales themselves are subject to a fair bit of wear and tear, and in my experience despite, in the UK at least, being required to be regularly calibrated etc (don't start me off about scales in more far-flung destinations!) can become less than 100% accurate although I can honestly say that there would be no malicious intent in this - just fact of over-use!!
Check in staff are sometimes but NOT always, incentivised to collect excess baggage charges;For some flights, the weight of an aircraft on a given day may be critical to the airline for whatever reason (cargo booking,full flight,air temperature,proposed flight route - basically anything that affects the flight performance) and the airline may be actively trying to reduce weight for that trip. Frequently it is just an issue of running a business - maximise the revenue and minimise the cost equals marginally more proft in an industry not known (especially in the current economic situation) of being a huge money-spinner.
Many - probably MOST - check in staff are employed by ground handling agents rather than the airlines themselves They are only TRYING to do their job and may have been briefed to be firm on baggage for a specific flight/period (and ,yes on some occasions this results in small benefits to the agent who braves the wrath of passengers to collect the most excess baggage revenue) I speak from experience when I say that suggesting that someone has excess baggage tends not to produce a mild-mannered and appreciative response!!.
My advice - smile & try to be friendly to check-in desk staff (it MAY just work!) politely ask for a re-weigh if necessary and if you are proved to be overweight and don't want to leave items behind, accept the charge with gritted teeth and chalk it up to experience!! You were, after all, warned on your tickets of your allowances and usually the adage "you get what you pay for" IS true, however hard to swallow!!!.....