I got that from Britax. Had a lovely conversation with a rather rude woman there when I was researching bigger seats for DD2 about 5 months ago. She kept refusing to give the weight and age limits for the seats, would only give me the shoulder heights! lol Ended up speaking to her supervisor too. End result was that they were changing the guidelines from ages to height.
This is from Safe n Sound's Facebook page re height markers after quite a few people were worried about having to forward-face their babies too soon,

Safe-n-Sound: Hi ladies, we understand that this is confusing and as a result we have revisited this issue. If your child reaches the shoulder height marker to move forward facing and are under 12kgs that we are confident that you can keep your child in the rear-facing mode up to 12kgs. If your child is above 12kgs and still under the shoulder height marker then they can still confidently remain there until they reach the shoulder height marker to move them. Please feel free to call us if you would like to discuss this in further detail.
9 December at 14:52

They have also said that the seats have not changed in construction since the introduction of the height markers and are tested to the same weights as prior to the change, for forward-facing and rear-facing limits. This was back in December though so for any newer models it would be worth checking. They are usually very helpful on their Facebook page.