Like Chinoiserie in the 18th century, with this cultural borrowing — in this case, of a bird found across most of Europe — the designers using these symbols are not engaged in, or particularly renowned for, presenting an accurate portrayal. These design elements are used to invoke a certain otherness (in this case Englishness, presumably) for an audience comprised of only a tiny minority of ornithologists. That is, the mostly Japanese audience likes the striking blue bird and the word 'robin', and wouldn't know a European robin if it landed right in front of them — the only reason I do, as someone who knows nothing of most birds, is that they visit my garden in Europe.

The illustrator likely just found that the blue bird just looked better in their sketches.