This is a fairly typical problem when trying to track and correctly identify new ships - as I do for a living!
LR Fairplay are the body responsible for issuing IMO numbers. However, they do make mistakes and occasionally a new vessel - one that is yet to be delivered - will be given two IMO numbers. Now, I don't work for LRF (I actually work for a rival company) but I can only assume that they have very similar problems to those I have when trying to gather information from shipowners or shipbuilders. First of all, it is a bit like trying to get blood out of a stone in some cases (though LRF do have the unfair advantage of having sole rights to the IMO number, so owners are OBLIGED to send them data under IMO regulations). However, whether it is sent to them correctly and unambiguously is another matter. It is quite likely that in this instance, as with many others I have come accross, that the hull of this vessel was being built at one yard (probably Eastern Europe) but the actual building contract was placed elsewhere - Shipyard Peters in this case. Consequently, if the initial information is limited, it might appear that two ships are being built when, in fact, it is only one. Two IMO numbers were initially issued, but when it became apparent that there was only one ship, one number was cancelled and thus appears as 'never existed' on the LRF database. The correct number will be the one appearing on the ship's hull.
A bit of a long winded explanation, but I hope it helps.
Phil