the time my message is read by someone

i’ve been wondering how this works or tryna see if this is a problem? so with the time someone reads my message. let’s say if someone read my message at 3:54 when i’ll double check it, it would say a whole different time (a time after the original read time) usually when i see it shows a whole different time that person texts me like if apple is showing me when their clicking and re-reading my msg but sometimes when i see the time change on the read receipts i don’t get texts from that person. i thought it might be everytime someone goes to type to you it’ll update the time they re-read it but idk can anyone tell me?