Why is it that Marx implies American settlers had proletarian backgrounds in Ch.33 of Capital, but Sakai asserts they were far from it?

Marx:

Today's wage-labourer is tomorrow's independent peasant or artisan, working for himself. He vanishes from the labour market - but not into the workhouse.

Sakai:

A study of roughly 10,000 settlers who left Bristol from 1654-85 shows that less than 15% were proletarian

many English farmers and artisans couldn't face the prospect of being forced down into the position of wage-labor.

Is it the difference of time periods? I just noticed now that the time period Sakai is talking would be a pretty early period of colonization, wouldn't it? So it may be that by Marx's time of writing (late 1860s-early 70s?) it was proletarians headed to America and had been in recent historical memory?