I was out shooting this weekend and I normally shoot three fingers under.I thought I would try shooting split fingers just to experiment a little.I noticed the bow seemed to shoot more quietly and I was consistently getting good groups either way.The only difference was I was shooting about 8 inches higher with split fingers.So could someone tell me why this is?

I'm not an expert but I'll offer my experience.

I anchor at the corner of my mouth.  When I shot 3-under, this moves the arrow (the nock end) up higher;  while my finger is still at the corner of my mouth, the arrow nock is now above my index finger.   When shooting split, the arrow nock is lower in relation to my anchor point.

I anchor with my thumb knucle on my earlobe when shooting 3-under.Maybe the differnce is my anchor points when shooting these two styles.

anchor is higher.  I have to change the nockset when I switch to three under.  Only shoot split now on longbows.

If you didn't change how you anchor or aim, then shooting split versus three under drops the rear of the arrow. You will find your point on distance is probably 10-15 yards further when shooting split finger.

As far as anchoring when I shot split I anchored lower using my middle finger at the corner of my mouth.I did lower my nockset down still hitting where I look with 3 under and still higher using split.Is it more critical with a longbow than say a recurve?

No lunchbucket NOT any different LB or Recurve.  I suppose folks are getting tired of my "theory."  1/8 to 1/4" change in your nock location at anchor will produce about an 8" error of the arrow at the target (20 yards).  If  you change from 3 under to split and use the exact same references at anchor, you lower the arrow nock a finger width, which is more than 1/8 to 1/4 Inch.

Arne

Thanks for the tip moebow.I was using different anchor points so that could be the difference I was experiencing.Actually I just noticed your picture under your user name.That is how I was shooting with split fingers.With three under I was anchoring a little higher.