Odd lots used to be second-class citizens: they did not update the tape in the same way and were long excluded from the protected quote. As share prices rose, odd lots became a large share of trading volume, which distorted what the consolidated-tape showed.
That is why the odd-lot quote was added to the tape and why a mixed lot — say 250 shares — is handled as a round lot plus an odd lot at many venues.
Example: buying 3 shares of a $3,000 stock is a $9,000 trade and an odd lot; buying 100 shares of a $4 stock is a $400 trade and a round lot. Only one of them counted toward the displayed nbbo under the older rules.