*Sigh* wrong...wrong...and wrong...
Religions that are divergent, such as your Judeo-Christian-Islam religions (all which "surprisingly" came from the same region....) A major influence on these Abrahamic religions was Zoroastrianism .Then your Hindu-Buddhist connection (surprisingly again (sarcasm here) they themselves acknowledge are related (buddhism was founded by an originally hindu) however, the metaphysical basis of hinduism -> christianity ends at the fact they both acknowledge a higher power of some sort, there is relation between the two faith systems, Vedic roots and Abrahamic, though the causative factor of this can be attributed to cultural contamination and cross pollination of religious ideals . Then you have your Celtic born religions divergent from druidic/nature bases (wicca falls in here), your native american religions, also nature based, but completely different in base from the Celtic nature religions.
An example of religious cross-contamination can be seen in the Tao/Buddhist/Native mythology mix found in china. While no particular "belief system" is defined by the en mass belief system by most casual practitioners of buddhism/taoism in china, it can be seen that it contains many aspects of both religions as well as chinese mythos (demons/ghosts), funeral rites are a great example here. Chinese burn Joss paper items (money, material items, etc fashioned from paper) however, this is distinctly in the face of buddhist anti-materialistic beliefs, although many of the practitioners of this funeral rite would state buddhist beliefs.
There is no strange coincidence or unexplained reasoning behind why many religions seem similar. It is not odd, or surprising. Religion can be viewed no differently as a language, since it is transported around cultures/geographies by communicative tides. Just as the local language may contain geographical (and even with geographical bounds, cultural dialection) dialects, so do religions.
Syncretism is the word for this. Syncretism occurs when a disparate school of thought/language/culture/art/whatever meet in a cultural locale. To rectify this disparity a natural syncretism occurs, it tends to meld over time ideas from each of the opposing ideals into a single school of thought that is unopposing to either (although this often is distasteful to those who prefer absolute purity in their ideals) During the spanish inquisition this was a common occurrence, many of the opposition to catholicism would incorporate ideas of catholicism while at the same time resisting conversion to the faith, this however was often seen by the inquisition as conversion thus saving the life of the syncretic convert.
In this wordy post, I'm simply saying, there is no surprise that religions often seem to have the same basis, most of contemporary religions come from about 1 of 4 or so base belief systems that have merged, mixed, and become what they are today.