Old bricks change colour with weathering
A new brick can look too bright when first laid and settle down later. Samples should be compared dry and in natural light.
Mortar can make the wall look wrong even with the right brick
Joint colour, depth and profile can change the apparent colour of an elevation. Very strong modern mortar can also be inappropriate beside softer historic brickwork.
A drawing can make a detail look settled before anyone has checked the existing building. Measurements, drains and structure still need confirming on site.
Toothing is not always possible
Where old brickwork is irregular or the new wall is on a different structural line, a straight movement joint may be cleaner and technically safer than forcing a false bond.