School readiness is not only based on numericy, reading etc,etc... but is also about Emotional Maturity.... so maybe their recommendation is not soley based on work, but also based on your childs emotional maturity. My child was due to start Grade 1 next year and I was advised to hold her back (I thought I was a terrible mother that wasnt doing enough) - but was explained to me that rather she start school in the middle, than at the bottom, coz she will fight her entire school career to stay ontop of things if I forced her into Grade 1 next year.
Maybe google EQ and IQ - and dont force your child if the school readiness already states that your child is not ready - you dont want your child battling their entire schooling years and hating school.