I suspect there might be a couple of issues at play here.
Firstly, do the box office figures here represent the takings at all Italian cinemas, not just the primera vizione showings? A lot of the lesser gialli went directly through independent distributors to the smaller provincial cinemas or were intended for cheaper second or third run showing. I'm not 100% convinced that the figures you get in the official documents include this kind of information, so the figures are skewed towards the 'prestige' productions that were seen as being more appealing to an urban, educated audience (where the first run cinemas tended to be). Box office figures in the 70s are even trickier to collate than in the 60s because the distribution networks broke down, and most reliable sources (ie Dizionario del cinema italiano) don't include anything about a lot of films. This also creates a statistical anomaly in that you'd have to sell a proportionally higher number of tickets outside the primera vizione - which charged more - in order to make the same money.
In addition, as has been mentioned a lot of these films made more money in foreign markets (although I believe they weren't quite as successful as westerns and spy films were back in the 60s). Don't forget that a lot of the time they were partially pre-financed by distributors working in foreign markets, so the costs were reduced even before they'd been made.
And, furthermore, they were cheap to make. They didn't have to be huge box office smashes in order to make a tasty sum for their producers. If Lo Strano Vizio bought in 600 odd million and was still only #70, well, that was probably still a 300% profit (finding budget info is even more tricky than box office info).
Most of the directors who have been asked about these films refer to them as being hits, from Sergio Martino to Maurizio Pradeaux to Lenzi. So I'm not sure that the fact that they're not listed in the box office charts means that they didn't make a good deal of money. Although I doubt they made as much as the westerns and spy films made by the same guys!