I see three questions beneath your one question, and I will handle them each separately.
Are Calvinists allowed to disagree with Calvin?
First of all, they get their name from him because they are believed to be in accord with him on most/all doctrine, not because they get their doctrine from him, although he is of course a highly esteemed and respected theologian. So in that sense, if Calvin taught something different than Calvinists, who cares? Many of them would care, but more because they want to know why they hold a different position than he does than because they need to agree with him. He is not a prophet.
The most important question for a Calvinist is not, "Does John Calvin teach limited atonement?" but rather, "Does the Bible support limited atonement?"
Do Calvinists disagree with Calvin?
Calvin's views of limited versus universal atonement are controversial. Both sides of the debate claim him. The quotes you provided in the question are sufficient context for seeing why he's claimed by the "universal" side. The article John Calvin's View of Limited Atonement by Roger Nicole concludes, after examining both sides' claims, that Calvin actually held to limited atonement and gives several reasons, mainly having to do with Calvin's strong covenantalism, trinitarianism, and belief in penal substitutionary atonement. Calvin also often stresses that Christ secured blessings "for the elect." None of it is a slam dunk, but Nicole says one passage (from a treatise written against one Tilemann Heshusius) "deserves special attention":
I should like to know how the wicked can eat the flesh of Christ which
was not crucified for them, and how they can drink the blood which was
not shed to expiate their sins.
His strongest argument, in my opinion, is this:
A historical difficulty appears when we attempt to explain how
Reformed thought moved so quickly from Calvin’s alleged endorsement of
universal atonement to the very emphatic support of definite atonement
by all but one or two of the delegations at the Synod of Dort. What
happened in these fifty-five years to cause the Reformed community to
make such a drastic shift?
But ultimately, I don't know if Calvin believed in limited atonement or not. There are two confounding issues here:
Calvin used Biblical vocabulary and left himself open to interpretation on some issues in much the same way Scripture itself is open to interpretation.
The controversy hadn't even happened yet! How could he have defined his position on a debate that hadn't taken place? So we're left guessing as to his position. Either his thought is more consistent with one position or with the other, but the question hadn't yet been formally considered. (However, the Wikipedia article does mention that there is support for the doctrine in theologians predating Calvin. But just like him, they didn't seek to define their support or opposition to limited atonement -- they merely held to a theological system that supported it and that showed in their writings.)
Why do Calvinists hold to limited atonement?
From a theological point of view, Calvinism is a systematic theology. John Hendryx of Monergism.com says that it is impossible to deny limited atonement and at the same time affirm unconditional election. R.C. Sproul says that four-point Calvinists always, on investigation, turn out to be "no-point Calvinists." The "L" in TULIP is one cog in a well-oiled machine, as they say.
From a historical perspective, as with all confessional forms of Christianity, time revealed new challenges to the system that needed to be answered. Jacob Arminius placed himself in opposition to Calvin and the system of theology expressed in the Belgic Confession and Heidelberg Catechism. The Dutch church needed to figure out, together, how to respond to his challenge. So they drew up the Articles of Dort, now remembered for giving us the five points of Calvinism, which answered the five points given in the Articles of Remonstrance. The development of the doctrine of limited atonement was simply a fleshing-out of the theological system developed by Calvin and his successors.
