I have both dyslexia and dyscalculia and yet still managed and A* and B on my GCSE respectively and now training to be a secondary school teacher.
It doesnt mean we're stupid. Nor does it mean we're incapable. It just means that we learn things in a slightly different way to other people.
When a person with dyslexia reads, its not that they dont know how to read, its more that they cant actually see the words properly. Often the words move around, the lines can sway up and down, for me, the words will flash, making it difficult to focus properly and get the letters in the correct order. This is why coloured paper or overlays is often used, it can help prevent that flashing effect. Others will experience things like the letters swapping over with each other, this of course will affect their spelling.
Dyscalculia is another whole bucket of fish. I can't count backwards in my head, I have always struggled with timelines. I often struggle to see patterns. No matter how hard I study, I cannot seem to make this work. However, I have sat down and learnt all the step by step processes. I can do maths, give me a piece of paper and I can tell you the answer to a question. In my head though? The numbers get jumbled up as in my head, a number has no value. If you were to speal out a series of numbers to me and ask me to put them in ascending order without writing them down, I couldn't do it. An 8 is equal to a 4. Its difficult to explain to someone who doesn't have it themselves. Its the same with clocks, show me a clock face and I can easily tell you what the time was an hour and 20 mins ago, in my head that clock gets all jumbled up and the numbers don't appear in the correct order. This is nothing to do with 'lack of learning' or 'lack of teaching', I have tried, believe me I have tried, there is nothing more embarrassing than being unable to join in a new years countdown because I can't seem to get those numbers in order, I can count up but not down. I can add, but not take away, I can multiply, but not divide. It has caused many tears and frustration over the years. I regularly get dates and times mixed up, this has caused me to become quite obsessive over my calendar and filofax. I dont ever use my bank card because I struggle to keep track of numbers, I can only deal in cash.
But again, it doesnt make me incapable. I simply had to spend more time than me peers learning Maths GCSE, I went to extra maths lessons every lunchtime and after school every other day, plus every school holiday for 3 years. I got full marks on my coursework, I did well at the calculator test, I got a low D on my non-calc and yet still came out with a B and that was because I had learned the processes. I just couldn't do it in my head. The B to me, was an A* to someone else. I worked harder than anyone else in my year group and so to this day, that B is the greatest achievement in my life, alongside passing my numeracy professional skills test first time.
Dyslexia/Dyscalculia are real things. To be properly diagnosed, you sit a psych exam, this exam drains you completely and I remember during mine I just sat there and cried in front of the bloke because I felt so useless and angry at myself for not being able to answer what I recognised as a simple question and that was when I was 17, it still stands as one of the most exhausting tests I've ever done because they are designed to really test your limits. A lot of people are self diagnosed or diagnosed by a parent or teacher without any real backing.
What you're mixing up is that often people see it as an excuse. It's not an excuse, it's a reason to work harder.
There is a difference in those who need extra help in lessons and who are slower at learning, to those who need a question asked in a different way in order to help them to understand.