Could they have deflected my shot with a .243 100-grain bullet?
Absolutely anything from grass, twigs and light bushes to heavy raindrops can, and sometimes will, deflect a rifle bullet.
And the closer the obstruction is to the rifle, the greater the deflection by the time it reaches the target.
American magazines used to talk of “brush buckin’ bullets” but in fact there is no such thing, as one of them demonstrated when they ran some serious tests about 40 years ago, shooting at a full size picture of a Whitetail Buck through various kinds of vegetation.
They tried everything from high velocity .22 centrefires up to ponderous, heavy bullets in .45-70 and though big and heavy beat small and light, none of them would penetrate reliably in a straight line through even light bush.
A few years ago my professional hunter in South Africa told me to shoot through a screen of long grass at the heart/lung area of a Blue Wildebeest.
The shot went a bit high but I really don’t know whether it was deflection from the grass or 50 years of experience causing me instinctively to hold just clear of it.
If it was deflection, that was a 310-grain bullet at 2000 feet per second fired from a .400/.350 Nitro Express, which may put your .243 into some sort of perspective.
The simple answer is that we can never be sure; and when you are not sure, don’t do it.