Hi there! Sorry it took me a couple days to get back to you. I see why you were tripped up: if you look at the Tricks for core skills, you will see that each one also grants you +1 in that skill at the end of the Trick's description.
This means, effectively, that your core skills improve whenever you roll a critical success using them, just like secondary skills do. It is not worded this way explicitly in the rules because certain Tricks have requirements for a minimum modifier, and in playtesting there was confusion about whether the +1 to a skill was taken before or after deciding what Tricks you were eligible for.
If your secondary skills surpass a core skill, this will be temporary, because they are capped at +5. You also will start with higher modifiers in your core skills since the starting modifiers are based on your attributes, and secondary skills just start at +1.
Please let me know if you have any other questions! I am happy to help :)