I have also found captains who have their own theories that differ from the manufacturer recommendations.
When the aircraft manufacturer does their certification test flying, they need to meet two sets of criteria. They need to meet the certification requirements, but they also need to meet the brochure numbers that the marketing department promised to all the customers. They therefore do a series of climb/cruise scenarios to find the ideal cruise climb profile that maximises block speed and/or range and/or payload. They spend a fortune doing this, because the answers are worth literally millions to them. Not all this data makes it to the AFM. Some will be in a supplement, or a planning handbook, or whatever - you may need to check for your type.
But, it is very unlikely that your Captain has inadvertently discovered a better answer than the company test pilots who were paid to find the very best way of operating their machine. You just need to find the appropriate guidance material and lead your Captain to it!