It's just while it settles down & "beds in" your results. Remember that Googles results don't just come from 1 index, they switch between a variety of data centres which do not all contain the same data, so if 1 data centre has already had you on page 1 then it's a good indication that this is where you'll end up.
Having said that one of my sites did the same thing - page 1 disappeared, page 1 disappeared - & it hasn't come back for the last 2 months
