A 6.1 rating means Nielsen is statistically 95% confident that the true number is somewhere between 5.8 and 6.4. A 6.0 rating means they're 95% confident it's truly between 5.7 and 6.3. There's too much overlap in those ranges to make a conclusive determination. At face value and in definite numbers, 6.1 > 6.0. In numbers subject to margin of error, that's a statistical dead heat.
Also, those numbers are rounded to the tenths place. 6.1 could be 6.05 & 6.0 could be 6.04.
Yeah, it's a lot of "could"s there, but that's how standard deviations, confidence levels and margin of errors work in statistics.
Statistics was my 2nd favorite mathematics class after probability.