Hello Stephen,
An ingenious boiler counting system, but not the most accurate?
Reduction from 4 to 3 boilers. Could be that;
1. Remaining boilers were uprated with new burners, output maintained and space put to new use. eg additional aircon plant when changing from North Atlantic to blue water cruising.
2. Remaining boilers not uprated, full power and speed not required when cruising. Probably only about 2 knots top speed would be lost, and it is far more efficient to operate 3 boilers at 80% capacity than 4 at 60%. Space saving for other uses still a bonus as in 1. above.
Building a ship with three boilers from the outset could be an attractive option. Shaw Savill's Southern Cross had 3 Yarrow boilers (built by H&W, not Yarrows themselves, as per previous posts relating to Babcock and Foster Wheeler).
I sailed as a passenger on the Cross in the 1990s when it was cruising as Ocean Breeze. It could easily maintain it's 17 knt schedule on 2 boilers and over the course of a year, just 3 boilermakers sailed with the ship and re-tubed all 3 boilers, one at a time. A cheap, efficient solution without incurring downtime that no doubt contributed to the ship giving almost 50 years service.