The answer is a very simply one - so simple that it confused the lot of them! They were too busy trying to cover up the shooting of Sheila downstairs in the kitchen, by adding a spent cartridge case from downstairs in the kitchen, to upstairs in the main bedroom so they could present Sheila Caffell's death there as a one gun crime! They had to move one of the spent cartridge cases (and swap it) from the kitchen, there had originally been 4 spent cartridge cases there which had been loaded and fired via the anshuzt rifle, and another spent cartridge case from the gun that had been used to shoot Sheila in the kitchen! This latter bullet case was confiscated, and one of the other four was transferred upstairs to the main bedroom!
This in turn left a spent cartridge case short in the kitchen, where supposedly Neville Bamber had received four head shots, but now there were only three bullet cases!
This very serious anomaly was a cause for concern of Essex Police, until someone came up with the bright idea that there could have been 26 shots fired, and not 25 - with 25 spent cartridge cases which could be linked to the anshuzt rifle, and one spent cartridge case belonging to the other rifle!