[css-anchor-position-1] Specifically subtract the offsets. w3c#8059

Author: tabatkins

css-anchor-position-1/Overview.bs

@@ -923,11 +923,12 @@ to trigger automatic "animation" of the fallback'd properties.
 			be |el|'s styles after these adjustments.
 
 		2. If |el| has a [=snapshotted scroll offset=],
-			then shift |el|'s margin box's position by the offsets.
+			then subtract the offsets
+			from |el|'s margin box's position.
 
 			Also, if any of |el|'s [=inset properties=] are non-auto,
-			add the [=snapshotted scroll offset=] for the appropriate axis
-			to their values.
+			subtract the [=snapshotted scroll offset=] for the appropriate axis
+			from their values.
 			Recalculate |el|'s [=inset-modified containing block=]
 			using these shifted values
 			to obtain the |scroll-adjusted IMCB|.