It is possible the starter motor is below par, sometime a segment on the rotor goes open circuit and it needs to turn a smidge before it will run. In that condition it takes more power than it should.
However the simplest thing is to ensure all the power feeds from battery to the relay, and to the starter motor plus the earths for the battery and motor are clean and snug. Check also that the battery acid level is correct.
If in doubt try a couple of jumpstarts from a known good battery.
Poor connections anywhere in the starter circuit can lead to arcing in the starter relay making it unreliable by damaging the contacts.
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