LT3840 DC DC converter voltage drop and current varation

Hello,

We have implemented and testing of LT3840 DC DC Converter ( same as recommended application circuit in datasheet- low part count application). We are facing below issues, kindly guide. 

1. Out input voltage is 28 V and output voltage is set to 7 V. for lower load current, output voltage drop is minimum, but with the higher load current , the output voltage drop is higher from 7 V to 5.7 V at 8.5 A. Please guide how to improve voltage drop with higher load current in LT3840.

2. There is a variation in the actual load current and the taken load current. we have verified with the ammeter. for load of 1 A, the current drawn and shown in ammeter is 0.300 A. what is the reason behind this. this variation is shown at every load. we have done testing from 100 mA to 8 A. and the variations are random. for 8.5 A load, the drawn current is 3.5 A.

please explain.