Why does yearmonth return something different if I type in a date?

Question

I want to create a yearmonth of "2016 Jan" and am using tsibble. For some reason when I create the yearmonth object I get back the month prior. Here's an example:

> yearmonth("2016 Jan")
<yearmonth[1]>
[1] "2015 Dec"
> yearmonth("2016 Feb")
<yearmonth[1]>
[1] "2016 Jan"

I'm confused because if I use make_yearmonth I get what I'm expecting and can use yearmonth to verify:

> ym <- make_yearmonth(year=2016, month=1)
> ym
<yearmonth[1]>
[1] "2016 Jan"
> yearmonth(ym)
<yearmonth[1]>
[1] "2016 Jan"

So why does yearmonth return something different if I type a date, e.g. "2016 Jan", compared to using make_yearmonth?