I would need more information the film and agitation scheme, but offhand depending on the processing temperature, the difference between 9.5 and 10.25 minutes is not much when it comes to materially changing the lower densities (depending on the inherent shape of the toe for a particular film). So while one might expect a more gradual increase in contrast with a small increase in time, rather than the slightly "kinked" curve you posted, it doesn't look out of line to me. Of course there can also be small measurement errors, plus the fact your test conditions might have significant flare which would tend to flatten the toe contrast.