You should be able to call their office and ask for the calculations used which will tell you whether the difference is in your acerage or in your percent of royalties. it will look something like this:
Mineral Interest: 0.50
Lease Royalty: 0.23
0.50 x 0.23 = 0.115
Gross Acres in Unit: 41.68
Total Acres in Unit: 221.02
Tract Factor: 41.68/221.02 = 0.1885802
Net Royalty Interest: 0.115 x 0.18858022 = 0.02168673
P.S. I have still not recevied my DO yet but hope to get them soon. I did hear that there were over 400 mineal owners in this pool and a lot of unidentified heirs hence the delay in sending out the forms but you would think they would have at least grouped them by owner....