Bit hard to tell, but unless you changed some major Thing about the Light on the Last Shot, i assume you used higher a Higher Gain Setting than on the lower FPS Shots.
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When used in Moderation, a bit of Gain in the right Situation (especially if otherwise the Frame would just be too dark, and you cant change that by other means) can actually give you much better overall Image Quality. At least that is true for Firmware 5.xx / 6.xx and Later on the 2.1, assume its about like that on the 1.4.
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While its ofc. always best if you just have enough Light to Shoot with no Gain at all, with the Newer Firmware you at least shouldnt be afraid to try bumping the Gain a little bit if the Frame is just straight up to dark.