It took me way too long to realize you were asking about historical background and not card art background, I was quite confused since the Chansey is clearly there 
It is 1 of 7 officially awarded No. 1 Trainer cards featuring a girl trainer instead of boy. There are 3 old back style and 4 e-reader style. I am personally partial to the e-reader though of course would be thrilled with either. This is an excellent guide for further information:
www.elitefourum.com/t/exact-number-of-boy-girl-trainers-old-back-new-back/28547/1