I did expect a higher IQ score, so I am not sure if I am clutching at straws or if I should have a genuine issue with the difference.

I have 4 DD's and DD1 tested as HG no quirks,grade skipped, easy going now studying medicine, scholarships, easy run.
On both DH's side and my side are family members who have tested as gifted, neither myself or DH have ever tested, DH would score quite high, my abilities are in other area's. Most cousins range in the MG-HG, even those family members not tested have some quirks and definite strengths.Most of both families are happily underachieving, as long as they have access to a good book pile they are happy iykwim. Over achievement is not a family trait on either side.
DD2 and DD3 I just recently tested,DD2 did dismally and we suspect she has some learning issues. School has never been easy for her and she is working at her age level, we think she is a VSL with auditory issues,current auditory testing has been delayed due to ear problems, she has had extensive testing in the past but was perhaps too young? She has some eye issues as well.
DD3 who is the child who got the higher WIAT is 7 and has been diagnosed with selective mutism, she has no eye issues nor auditory issues and is very much a auditory sequential learner. School is easy, basically she learns quickly, puts in minimal effort and stays at the top of the class, we have not extended her at home.Instead we have focused on her socially which means she dances 4 days a week & does martial arts 2 days a week, she has no time for homework or any learning outside of school but her social confidence has improved. She is in a grade 1/2 class works at both levels.

WISC was in the average, WIAT in the superior range

I really thought her WISC would be MG and her WIAT would show she was perhaps under performing as we pay more attention atm to developing her social skills vs her academic, we are not extending/coaching/teaching her at home, it has been a deliberate choice to not do so. I would confidently describe her as not working at full capacity.

She receives moderate access to extension activities at school and is considered a capable student with verbal limitations, generally works at the higher grade level (composite/multi age class).Her teacher is very nice and is not at all pushy, much attention is paid to her social needs, so I don't think her teacher is overworking her either.
She can't ask questions (hasn't got the verbal ability ie could ask at home but won't in a public setting) so I don't see how she could be working above her IQ level.

I just don't see her as capable of working above ability, I would imagine to achieve above ability it would require a large amount of effort from home and school, it would also involve a child who has a different personality to my DD. She puts in minimal effort, keeps up with older peers and doesn't ask or seek out help, she is a compliant child, who is not in a rush academically.

Should I ignore the current assessment or take it on board?

Even in the WIAT she scored highest on her least favoured subject and scored the lowest on her favourite.

Is it normal to get a difference like this?