Circular saw won't cut vertically
Despite what you might believe, those adjustment levers on most tools are not calibrated very well.  Even on my table saw I regularly use a simple plastic 90-degree triangle to ensure my blade is aligned to the table.  This is part of owning any tool and it's worth a few moments to verify everything is set up properly.  A more expensive tool will simply buy you more time between calibrations but all tools should be checked with regular use.