T-Bill interest calculation

Terry, I have purchased several 26-week t-bills over the past year as you have suggested on WGN. Each has been reinvested. At the time of reinvestment, my bank account was credited an earnings/interest/yield(?) amount close to what I expected (as per the discounted purchase price), but always a bit more or bit less than the expected amount. Can you tell me why this would be? And can you explain interest rate vs. yield rate? Why are the two values different? I’ve tried to find the answer on Treasurydirect.com and by phone with no success. Thanks for your reply.