Investment account

Mr X purchased 500 shares Cost of original lot ,purchased on cum right basis- 20,000(20 per share) 
Cost of shares after becoming ex right- 19000(19 per share)
Mr X now sells the shares at 3 each.
SOLUTION : Bank a/d DR 1500
                     To Investment a/c 1000
                     To Income a/c 500
Query: Why isn't the whole amount credited to the Investment account? What is the need to credit 500 to income a/c? Is it an alternative treatment?