According to the 'plans parcellaires' of the area dating from the end of the 19th century (which can be viewed online on the city of Paris website), there was at the time a 72, 74 and 76...but no 78! What seems to have happened therefore is that at some point after this the 74 and 76 were demolished and replaced by a single building which was given the number 78, but why this should be the case remains a mystery!
Such cases are not actually very rare, but normally the gap between the two numbers can be explained, for example, by the presence of a newer road which has cut between them. Even more common still is the newer, larger building which takes the place of several smaller demolished structures, adopting all of their previous numbers.
In London, a famous example provides the flip side of this phenomenon. Two houses at 23/24 Leinster Gardens were demolished in the 1860s to provide ventilation space for the newly installed Metropolitan Railway, but as this was something of an upmarket district, the residents insisted that the space be filled with a fake facade. These 'dummy houses' still exist today, and of course have been the address given by many scammers over the years!
The city, with its constant destructions and reconstructions, is riddled with wormholes. By nature, its history is non-linear, so should we be surprised to find such inconsistencies in its labelling?