The only reason I've ever been able to come up with myself for this is maybe they want to leave space between the numbers of different batches and eliminate the possibility of batches being numbered in a different series which could be used for a future order of vehicles. For example the last batch of Tridents Lothian ordered were originally numbered all the way up to 705 I think? Which where then renumbered and the 700-800 series went to the Wrightbus Volvos
Maybe so that when they fill up the fleet, they can buy a batch of 4 singles numbered 99-100 and 199-200 then a batch of 15 deckers (299-300, 399-400, 499-500, 599-600, 699-700, 799-800, 899-900 and 999) and nicely fill up the gaps. (I'm joking, they very likely won't) - but mind you this isn't all that new, 599 and 600 were left out when they bought the tridents, however later 629 and 630 were given those numbers as part of all the renumberings. Having some wee gaps could com in useful in the future who knows.
My only personal theory was them calling Volvo/ADL and latterly Volvo/MCV saying "I'd like to order 98 please" hoping they'd go "well a press release saying Lothian order 100" will look great so have 2 more on us but it's not worked
