What is the exact line of code that you are using for this part of the cart display? It seems that there is a problem with the way that you have it. I can get the exact same result as you are getting if I just take the total and multiply it by .19. Here is the code and the results I get with it:
item:119 total:119 tax:22
<?php echo WA_eCart_DisplayMoney($eCart1, ($eCart1->GrandTotal() * .19)); ?>
item:119 total:119 tax:19
<?php echo WA_eCart_DisplayMoney($eCart1, ($eCart1->GrandTotal() - ($eCart1->GrandTotal() / 1.19))); ?>
If you have any further trouble with this please post back with the cart display page and the WA_eCart folder from the root of your site. I can do some testing with your files if need be.