Jantti is variant of Jentze. Jentze is German surname. On 16th century a vicar called Laurentius Laurentii Jentze moved to Rautalampi, Finland. Finns found it a bit difficult to pronounce Jentze so it became Jantti (with a dots), a Finnish surname.
At the beginning of the 20th century many Europeans, including many Janttis moved to North America. There are different family-branches in North America, but every Jantti moved from Finland are somehow related to each other. At least I think so..