The WIAT is no better and no worse at capturing math ability in a first grader than most of the other common comprehensive achievement measures (WJ, KTEA, PIAT); it is a solid instrument. For a first grader to score 146, she would need to know basic arithmetic operations, and multi-digit addition/subtraction, and be able to apply slightly easier skills.
Oral language includes several vocabulary or vocabulary-related measures. Listening comp has two components: a vocabulary task where she is orally-presented with a single word, and must select the picture which best represents its meaning from a field of four; and a listening task where a series of brief recordings is played (beginning with short sentences, and potentially progressing to lengthy paragraphs), each followed by one or more comprehension questions. Those with a good vocabulary tend to do well on the first component, and those with good attention and listening memory on the second. Oral Expression includes two components with heavy vocabulary skill-loading. One is a picture naming task, with brief verbal clues, and the other is a retrieval fluency task, where she names as many words in a specified category as she can in one minute. The last OE task is sentence repetition, which, again, is to the advantage of a child with strong working memory.
Without referencing her actual cognitive scores, her achievement profile suggests that she might be strong in working memory, as the academic strengths are closely-related to WM (mathematics, and several aspects of OL as assessed on the WIAT).
As to the PG-ness of her mathematics: 146 does meet the DYS criteria, so I think it's safe to say she's scoring PG in mathematics!
