I've noticed this before, essentially Eagle creates the offset borders of polygon fills with lines rather than curved traces. In some circumstances, this is visible. On small vias, the result is impossible to distinguish from an actual circle. I've also seen this effect appear on large, sweeping curves; it seems to be triggered on a raw x or y division limit. Anyway, the reason for this is you could have thousands of tiny vias on a board, and making all the polygon holes smooth would require many, many tiny trace segments.