SysTick accuracy in STM32
Crystals are usually much better than 50ppm accuracy ... BUT ... crystals have two resonance modes - series resonance and parallel resonance (with an impedance plot, you will see the impedance rise towards infinity at parallel resonance and fall towards zero at series resonance). 

Now the importance of this is that the two resonant modes are usually a few hundred PPM apart, and only one of them will be at the marked frequency! 

If you just buy an "8 MHz" crystal without paying due attention to the fine print, you may get one cut to 8 MHz in the wrong mode; and your oscillator will be several hundred PPM off tune. 

(The parallel resonance is also tunable over 50ppm or more, so it is usually specified at a given load capacitance).

You'd think most professional designers would pay more attention and select the correct crystal - and you'd usually be right - but I have even seen some high-priced digital audio equipment where this mistake was made! 

Or perhaps component purchasing found a "good deal" or it was simply decided that for a budget evaluation board, price was more important than timing accuracy...

But anyway my guess is that the crystal is operating in the wrong mode to produce a 300ppm frequency error.