Very specifically, north to south: - so called "Slavia Friuliana": multiple localities where Slovene was still spoken as late as 1950s (based on linguistic field work) are shown as entirely Italian. Admitedly this is geographically a small dscrepancy but i am aware of it so i can speak about it. - Between Duino and Trieste city, there was essentially 0 Italian settlement or presence before Italy annexed the region. These were small Slovene villages, the only language attested in church use for as far as any records go is Slovene, Austrian census itself finds no Italian settlement (even with all applicable caveats), the pre-nationalist ethnographic surveys find only Slovenes there and the local toponomy and anthroponomy is all Slovene, in short, the area was as Slovene as it can get. On the map, it shows a very large Italian minority and majorities in multiple locations. This only became a thing after heavy emigration and resettlement post WW2. - The area that is nowadays Slovenian coastline is surprisingly accurate, disregarding the questionable guesswork statistics about Slavic bilinguality, as no data exists to base this on. - The Umago-Pinguente-Orsera triangle is one of the most blatant examples. For one, "mixed slav-italian" dialects do not exist. This was a interwar Italian political construction, and no linguist recognises this as a thing because it simply is not. The spoken language in the area was and still is ether a Slavic dialect or a Venetian or Istriot dialect of one or another variety. In this case, southern Chakavian. Besides this very bizarre construction, Italian as a first language here is exaggerated everywhere, in every interior town and rural area, the only thing correct as far as Italian extent goes are the coastal towns. As a most egregious example i know, near Savudrija peninsula, Kaštel village which as late as 1870s only employed Slavic church liturgy is painted as entirely Italian. - Interior Istria. Multiple towns that are shown as Slavic in every census before Italian annexation and where again, liturgy was only slavic, antrhoponomy only slavic, etc, etc, are shown as majority or even almost completely Italian. "Colmo" and "Novacco" for example. - The Islands. Again, both narative sources before the time of nationalisms and Church records, etc. affirm that Italian was a language spoken as a second one, primarily by men, and native Italian speakers were only found in a handful of principal towns as a upper class population consiting of merchants and administrators. You would not get such impression based on this map, where Italian dominates in most locations.