Dear Umesh,
You are right that profit on sale of fixed asset is a capital reciept.. The answer to your question lies in the basics of passing the entry...
See when you purchase an asset and when it is put to use, you start providing depreciation.. As a result the asset value comes down to a value called book value by providing depreciation and charging to p&l
So when on a particular date you decide to sell that asset you may earn a profit or may also loose... for e.g... suppose an asset whose purchase value was INR 100000 and till date you have provided dep. of INR 50000... so now book value will be INR 50000. Now when you decide to sell that asset at INR 60000, you will be making a profit of 10000 which will be credited to P&l... Why its done????.... See all the years you have provided dep in P&l and written off some capital value of the asset.. So now when you sell it at a value more than book value, you are recovering nothing but the already written off value of the asset... Its because of this reason you credit back the p&l with the recovered amount in terms of profit on sale of fixed asset...
Hope you understood..... Any doubts feel free to ask..