I disagree with the response above as the appeal process depends from uni to uni and may or may not work for you. At my uni, they make it explicit that appeals for grades won't work and it's extremely rare for a case like this to go through. Regardless of this, any advice regarding appealing processes should be the first option and likely you'll get a negative result.
Each uni marks the exams/honours projects/coursework differently. They try to "balance" out the grades. For example, if many students did exceptionally well in coursework, they will be much stricter in exam marking to "level off" the grade. This is to avoid a potentially much higher average than the previous year.
Secondly, we have many emerging technologies such as ChatGPT which can be used for assistance in writing or even completion for our coursework. It's very likely that most students nowadays will achieve higher scores on average in courseworks and assignments. Other unis have taken strict measures to prevent this from happening by strictly marking courseworks and assignments (including honours projects), knowing that students will use assistive technologies to complete their work or bring the average down to what it was the year before.
I am assuming that if you put so much effort and despite your dissertation deserved a 70+ you still got a lower grade, it might not be your fault. The marking process was too picky and strict. In my case, I was expecting a grade close to 70 (a bit above or a bit below it) in my dissertation. I got 85% in the end which is MUCH higher than expected. I know my project doesn't deserve such a grade given that my project didn't have any major novel elements to be graded as such. I had another expectation of getting at least 55% in one of my exams (me being strict with this expectation) and thought any mark below 50% would be impossible. I ended up with a 41%. I found out from my peers that they got much less than expected in their exams and much higher in their dissertation projects. This further reassures me that whichever grade I will get, won't reflect my "true" skillset and that grading is no longer what it used to be.
It's not your fault. Thanks to our technological and AI advancements, more and more institutions will try to curve the grades (scale them secretly) to keep a competitive average. You may deserve an 80%. If most get 80%, then your grade may be brought down to the 60s.