My little, who is 8, can act much more mature than her age. She's able to txt, but has spelling problems unless an older alter or myself helps her. However, without help, she's able to process a lot more than normal 8-year-olds and comprehend a lot more. But she still has the general mind-set of an 8 year old and acts like she's 8.
Your little sounds like she's able to do things she shouldn't be able to do because of default. She may be going off of other memories. My 8 year old shares all the memories I do, they're just not as clear. Just like my childhood memories aren't as clear to me, but to her they feel like they happened yesterday. Your little might be doing the same and learning from things you do every day without you knowing it. It could also be that she's going off of instincts. You still have the body and developed mind of an older person, even if she has the mind-set of being 5. Your fingers still remember how to txt, your mind might auto-correct spelling, and so she can do these things as well. Another possibility is that she has the potential to be 8, but she either wants to be younger due to her main mind-set or something is holding her back. Such as, something traumatic may have happened at that age, and she's unable to move on from it even though she's smarter than the average 5 year old.
Either way, I highly doubt your little is lying. Hope this helps, and if it doesn't, hope this site in general does.