I understand the position that Christ was able to sin, but it was certain that He would not (higher decree). If "ability to sin" to be actual, implies that there is a possibility (chance) of failure or the validity of your claim is dismissed. Example would be Charles Hodge quote:

"If He was a true man He must have been capable of sinning. That He did not sin under the greatest provocation; that when He was reviled He blessed; when He suffered He threatened not; that He was dumb, as a sheep before its shearers, is held up to us as an example. Temptation implies the possibility of sin. If from the constitution of his person it was impossible for Christ to sin, then his temptation was unreal and without effect, and He cannot sympathize with his people."
Thus "chance" is not a conflation of "ability", but a possible outcome or consequence to the decision made.
Geomic