Children have different capacities to remember words they encounter in their reading books, but even if they are good at remembering common words by sight, they still need a really good knowledge of the alphabetic code for longer term reading and spelling.
I am trying to enlighten people that, for the most part, adults themselves apply their alphabetic code knowledge and phonics skills for reading unknown technical and longer words and for spelling longer words. If I am reading silently for my own purposes, I actually 'skip over' many such words which is a form of laziness - as I still acquire the gist of the text.
Sadly, many, many young people do a lot of 'skipping over' words and still give the appearance of having gained the gist of the text. This is all part and parcel of the prevailing teaching methods of many years where teachers are trained to teach children 'by a range of strategies' and where children and their supporting parents have had to apply a 'range of reading strategies' by default - because the reading material meant that children have had to guess the words - and the schools certainly did not teach the alphabetic code properly, thoroughly, or teach the skills of blending for reading, and segmenting for spelling, thoroughly enough - or at all.
Sadly, many schools do persist with the range of reading strategies which largely amount to guessing words from word shape, picture and context clues - and from multiple exposure to common words. There is always a group of children who apparently manage fine with this mish-mash of methods - but many children do not fare well - and certainly not in the long term as vocabularly widens in the reading material and silent 'skipping over' words kicks in.
It is a brave parent who tackles his or her son's or daughter's school - but it is important not only for your own child but for other children too. ALL schools should be teaching rigorous synthetic phonics for reading and spelling but the level of many teachers' knowledge and understanding of the teaching is probably not that great yet.
It is really not their fault, however, considering the demands made upon teachers and the climate in which they work. They themselves are not adequately supported by detailed rigorous synthetic phonics programmes which sustain the rigour of several years.
I provide a great deal of material and information which is free via my Phonics International website - helpful for both parents and for teachers. Most schools, however, do not even know about this material. If you ever need any practical support or moral support to address reading and spelling problems, please don't hesitate to visit the UK Reading Reform Foundation website at www.rrf.org.uk and, as I said, there is much free information and free material in unit 1 of www.phonicsinternational.com . If you need 'evidence' to back up your worries for approaching schools, see also this amazing website which is very heavily referenced www.dyslexics.org.uk .