As an older mum, that had very old guidelines, I get confused about a lot of things
I believe all babies develop at very different rates and that includes moving on to solid foods. My eldest was text book (for the time) 3 hourly feeds and solids at around 17 weeks, she followed the guidlines at the time.
Child 2, was an unsettled pita baby, never ever seemed to enjoy a bf or ff, was very forward in developmental milestones, at 16 weeks she did grab bread and she did eat it and cried for more. In a week, she went from that one taste of bread to 3 meals of mush and finger foods and was eating sandwiches and what I now know as BLW by 5 months. She slept through the night for the first time that first week too and turned into a happy baby. She also rolled at 12 weeks, sat up at 16 weeks and walked at 8 months, spoke early (but took forever to learn to read lol)
Child 3 was a big, slow lump of a boy, exclusively bf for a year, established solids not until 8 months and slept through 12 hours, with 2 naps from birth. He showed no interest in food at all, until 7 months ish, when he went straight into chopped food. He is the one with severe asthma, so maybe his body was helping him avoid allergens (in a hippy way)
I don't understand why people talk about calories, it is digestion that is the thing that can help. Solid food, will sit in the tummy longer and stave off hunger. They still get the calories from the same amount of milk as before (unless you get my no2 child, who stopped drinking milk at about 6 months)
I believe guidlines are good ad should be followed where possible (to stop the silly weaning ages and to give a parent an idea of what to do) but there will be exceptions to the 'average' baby
When my time comes to be a grandparent, I will tell my kids to follow the guidelines as best they can but to watch for signs aswell. I truly believe being ready to eat is developmental stage, just the same as being physically ready and able to walk is