by the expectation that the new salary should not be too much higher than the old salary
From where does this expectation derive, and what makes it a good barometer of sanity? On the face it just sounds like the same rationale used for basing raises on percentage of an employee's initial salary. "3% over your last job" is not fair.
The expectation derives from peoples lack of knowledge of what they're worth. It has nothing to do with fairness - it's about what they think they can get away with offering.
Based on experience, almost nobody negotiates their offers, and if you see people talking about their job moves, people are often excited about, say, a 10% increase.
That's why employers expect to be able to get away with anchoring it to your current salary.
From where does this expectation derive, and what makes it a good barometer of sanity? On the face it just sounds like the same rationale used for basing raises on percentage of an employee's initial salary. "3% over your last job" is not fair.