I am not sure what the "syncretic beliefs" patch in Zimbabwe and Zambia is supposed to represent. I am also pretty sure that Madagascar is not majority Hindu, although I did not bother to check.
Zambia's actually fully coloured in, while Zimbabwe is striped, anyways they're shown as such because the largest religions in Zimbabwe and some of the largest in Zambia, while originating from Christianity, hold sufficient differences to no longer really be Christian, but rather Christian-based religions.
Your map also woefully simplifies the African religious context. I'm pretty sure that Zambia and Zimbabwe are at least 70% Christian.
The first map shows both religion and, when applicable denomination, the second one shows just religions overall; while the majority of Zimbabwe does follow Christianity, the majority of Zambians follow churches that preach a syncretic religion that mixes forms of Protestantism with indigenous beliefs to the point that they're really no longer Christian anymore than Christianity is Jewish.
Also, syncretism =/= indigenous belief =/= syncretic religion. Madagascar's religion, for instance, is a type of veneration of ancestors which is also practiced by the local Christians. It doesn't necessarily means that they have an organized syncretic religion per se, like Santería. It's pretty hard to truly define the strength of each practice in this type of society, however, for sure, Chrstianity is the "religion of prestige". It's pretty much the same reality that we find in Shinto/Buddhist Japan or in the Americas with Christians who practice Santería, Voodoo, Candomblé, etc.
While their is no organized religion, their still is a major social difference between the indigenous beliefs and Christianity as even now there's still lingering feelings of Christianity being foreign (originally it would've been seen even more negative, as the religion of the conquering French).
I'd say that it's only around in the gulf area we can still find "true" native religions, but it's just a wild guess.
Assuming you're meaning the Gulf of Guinea, while yes, indigenous religions can still be found in the region (majority in Togo, second largest in Sierra Leone and Guinea-Bissau and minorities in several other countries), however the region has been influenced by Islam and Christianity for centuries, so many of the less organized groups and those in religious border zones will have been influenced by one or both.
Most of the African countries don't have reliable demographic sources anyway...
That depends on the country, on the one hand you have places like Tanzania, where apart from knowing that Christianity and Islam each make-up close to half (though I've never seen and legitimate studies saying either was a majority) of the population, we don't know much more of the mainland's (we do know Zanzibar's, since it does its own thing) since the Tanzanian government has refused to include it in any census or studies since the mid-60's, however other countries have ok to good studies on it, both via their own censuses and also through studies by outside sources, like the United States, NGOs, private institutions and others.