Why is one implant firmer than the other? Is it capsular contracture?

Scar tissue forms naturally around the implant, and the firmness you describe is a result of thicker scar tissue than what has formed in your left breast, as you describe it as being softer. Your surgeon is probably right, however you should keep an eye on it. Best of luck.

Just to be clear in our terminology, one implant is not firmer than the other, it is your right breast that is more firm. Or even more accurately, you have less implant movement on your right side than your left. This is capsular contracture, though by your description, it is mild (Baker 1 to Baker 2).

The scar capsule your body forms around any surgically-implanted object can either be thin, soft, and pliable, with a pocket size larger than your implant, which allows normal movement within the capsule, or it can be thick, tight, and uncomfortable. The thicker (more) scar around your right implant has contracted to a degree that has not restricted movement entirely, but which makes it feel ball-like as you move it along with the breast rather than within it.

Scar tissue can indeed soften and stretch as it matures, but at 11 months there may be very little of this to yet occur, even with continued implant movement exercises. If this obvious discrepancy persists, return to your surgeon. Additional surgery may be necessary to improve your softness, movement, and symmetry. Best wishes!

Each breast will always feel a bit different.  As for capsulate contracture, this is best assessed by physical exam and by your symptoms.