Small Discount Error

This is an odd one.

I have quantitiy discounts for one product (cost of prduct =
2.13).
Here is a quick breakdown:

6+    $    2.09  ;  
12+ $    2.07   
18+ $    2.04   
24+ $    2.02   
48+ $    1.99
Every discount "level" works fine (from strart to finish in the order)
except the last one, 48 or more =$1.99.

Here's the code from my product page:

<input type='hidden' name='discounted' value='Yes'>
                      <input type='hidden' name='cdiscount1' value=''>
                      <input type='hidden' name='cdiscount2' value=''>
                      <input type='hidden' name='cdiscount3' value=''>
                      <input type='hidden' name='cdiscount4' value=''>
                      <input type='hidden' name='volume1' value='11'>
                      <input type='hidden' name='volume2' value='17'>
                      <input type='hidden' name='volume3' value='23'>
                      <input type='hidden' name='volume4' value='35'>
                      <input type='hidden' name='volume5' value='47'>
                      <input type='hidden' name='volume6' value=''>
                      <input type='hidden' name='discount1'
value='2.09'>
                      <input type='hidden' name='discount2'
value='2.07'>
                      <input type='hidden' name='discount3'
value='2.04'>
                      <input type='hidden' name='discount4'
value='2.02'>
                      <input type='hidden' name='discount5'
value='1.99'>
                      <input type='hidden' name='discount6' value=''>