Difficulty is a rule in the protocol, not an estimate. At the end of each 2,016-block difficulty period the network compares how long the period took with the two-week target and scales the difficulty up or down, within a factor of four.
Higher difficulty means more expected hashes per valid block, so at unchanged price, subsidy and fees a fixed miner earns less expected bitcoin. The Gazette reports each retarget from block records on two explorers and recomputes the change itself.
A rising difficulty tells you the network did more work; it does not tell you who did it or why.