Your recollection of the 4 days estimate or 2 days safe decay is probably what is at fault here. If 6 days ago your arena rating was 1851, which according to our backups it was, and you had 2 days safe from decay, then you experienced 4 days of 2% decay, you would be at 1706.
You're welcome to track your rating more closely and plot it's rise and fall, but I don't think you'll see anything strange.
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Computing the probability that at least one of the following events will occur:
P(a or b ... or z) = 1 - P(!a and !b ... and !z)
Probability
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