Nobody explained why to the the dear boy. I am not an expert by any means nor am I a metalurgist but let me try to explain this to you. Steel expands at a rate of .000007" per degree F. Therefore, if you have a 1" diameter barrel which is nominal at 68*. At 100* that barrel would be (100-68)x.000007" or 1.00022". Now this is outside diameter which, although very small, will increase pressure at any contact points along the stock which will change the harmonics of the barrel. Now add this same expansion rate to the bore will cause the bore to become smaller in diameter which also changes the amount of friction on the bullet as it travels down the barrel.
Then you have to add the increased temperature from continued firing and there you have it - a change in accuracy and or point of impact.
But don't let that stop you from shooting in the summer. Practice, practice, practice makes you a better shot.