פירוש על בראשית 31:8

IF HE SAID THUS: THE SPECKLED SHALL BE THY WAGES. The meaning thereof is that at first Laban agreed to give Jacob the two appearances the speckled and the spotted — also the brownish lambs. Then he retracted and agreed to give him another color, and thus he changed it every year. The flocks, however, gave birth accordingly. This was not due to the power of the sticks, for he was telling them of the deed of the Great G-d who treated him wondrously each and every year, just as he said, But G-d suffered him not to do me evil.¹⁷⁸Above, Verse 7.
It is also possible that Laban changed his wages after the flocks became pregnant and gave birth in accordance with Jacob’s will, it pleasing the Creator to do so. [In this case, it clearly was not due to the power of the sticks.] And so we find in Bereshith Rabbah:¹⁷⁹74:2. “The Holy One, blessed be He, foresaw what Laban was destined to do to our father Jacob, and He created the form of the sheep to conform to the colors Laban was to stipulate. Thus, it is not written here, ‘If he said (amar) thus,’ but it is written, ‘If he will say (yomar) thus,’” [indicating that G-d foresaw what Laban was destined to stipulate to Jacob].

וילדו כל הצאן, this percentage could certainly not all be attributed to exposing the sheep to the peeled sticks.

אם כה יאמר, "If he said thus, etc." Although the original condition had been that Jacob should take the spotted and speckled ones, he did not honour his word but agreed to give him only one of these two.