By default, all fills in your project are set to be displayed as Vectorial Hatching, at View > On-Screen View Options.
If you turn this control off, fills will be displayed on screen in bitmap mode. However, when printing or plotting fills, the vectorial face will be used even if the on-screen display is set to bitmap.
The bitmap versions of all predefined fill patterns approximate their vectorial equivalents. If you change a vectorial pattern or generate a new symbol type pattern, you will need to create its bitmap version manually.