Using Version 2.00+ Build 130923 I entered the following formulas:

sum(1,2,7,'a') = {sum(1,2,7,'a')}

sum(1,2,7,'L1') = {sum(1,2,7,'L1')}

sum(1,2,7,'L001') = {sum(1,2,7,'L001')}

sum(1,2,7,'L001L') = {sum(1,2,7,'L001L')}

sum(1,2,7,'LOO1') = {sum(1,2,7,'LOO1')}

sum(1,2,7,'001') = {sum(1,2,7,'001')}

sum(1,2,7,'001a') = {sum(1,2,7,'001a')}

...and the result was:

sum(1,2,7,'a') = 10

sum(1,2,7,'L1') = 10

sum(1,2,7,'L001') = 10

sum(1,2,7,'L001L') = 10

sum(1,2,7,'LOO1') = 10

sum(1,2,7,'001') = 11

sum(1,2,7,'001a') = 11 <-- the chars are considered numeric until the first non-numeric string.

This is what I expected. I don't know why you would get anything else...