Appearance of this hymn in hymnals (percentage)

When I search for "Ein Lämmlein geht" there are two hits, both of which say "Ein Lämmlein geht und trägt die Schuld", but the first leads to the hymn with that first line, whereas the second leads to the variant with "Lamm" in lieu of "Lämmlein". When I look at the first result's authority page, I see among other things that this hymn appears to have appeared in every single hymnal published in the 1750s, then in a decreasing percentage of hymnals over the course of the next century, and finally in a very small percentage of hymnals over the next 60 years or so, and in none whatsoever since about 1920. I realize that a lot of the early hymnals in the database are German-language ones, and that perhaps more of them than of their English contemporaries have index data, but still I find it hard to imagine that the percentage in which it occurs is actually as high as the graph suggests. Is this just a byproduct of the Teutonic bias of the indexed hymnal set, or am I misunderstanding the meaning of the graph, and if so, what does it mean?