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I think part of why they won't disclose in general is because if they tell you that there's an offer of 100k, you'll just offer 101k. If they tell you there's a "strong above asking offer", there's a greater chance you'll just barge in at 110k or similar.
EA's I know just say they won't accept small incremental bids like that, it's just not worth the hassle.
There's no obligation from the vendor to take the highest bid. If someone finds out the current offer is 100k and offers 101k he'll just say "I'll pass it on but my client is happy with the current bidder but would reconsider their options if the increase was more significant", i.e. "offer 105k or go away".
I'm not sure there's any real regulation around these. In fact, I'm sure there isn't, since I've been in one of these and been told by an estate agent where my offer had to be once the envelopes had been opened.
I think part of why they won't disclose in general is because if they tell you that there's an offer of 100k, you'll just offer 101k. If they tell you there's a "strong above asking offer", there's a greater chance you'll just barge in at 110k or similar.