This is based on some preconceived notions I have so if my premises are wrong, please let me know.
I am not bound to all 613 mitzvot. I get this from here, and through a look at the Rambam's list. Some simply don't and can't apply to me -- and this includes negatives (a woman shouldn't wear men's clothing) and positives (that a Kohein should put on the proper garments for the service).
I know that (from reading related questions) there is some reward for fulfilling mitzvot I am not obligated in, though less than for someone who is obligated in them, but as far as I understand, I do not get a reward for mitzvot that I can't fulfill (unless one counts "learning about them" as equivalent), or even ones that don't apply to me that I DO fulfill (if I wear the priestly garments and perform the service, I can't imagine that I get any reward for kiyum hamitzvah - in fact, I would assume that it would be problematic).
Why then do I ask that my performance of certain mitzvot should be considered as if I fulfilled all 613? I say things like
כְּאִלּוּ קִיַּמְתִּיהָ בְּכָל פְּרָטֶיהָ וְדִקְדּוּקֶיהָ וְכַוָּנותֶיהָ וְתַרְיַ"ג מִצְות הַתְּלוּיִם בָּהּ.
The Artscroll translation is May it be your will that (insert commandment here) be as worthy "as if I had fulfilled it in all its details, implications and intentions as well as the six hundred and thirteen commandments that are dependent on it."
Why do I want to be considered as if I had fulfilled 613 mitzvot when some of them are not meant for me and, were I to fulfill them, I would not receive any reward (and it might even be a bad thing)?