Study the ShapeShift fee structure. We are getting the exchange rate from ShapeShift API, there is nothing we can do. But their fees are very clear and it is the miners fee only. So as less as you deposit the bigger is the impact due to the miners fee. For larger amounts you might not even take notice of the miners fee.
The shown approximation of their conversion is simply misleading.
Is is not misleading, it is simply how it is. There are miner fees for almost all cryptocurrencies and you have to take this into consideration. On the other side you are free to exchange your alts at various exchanges and deposit bitcoin with us.