Dear Melanie,
Thanks for asking!
The difference in survival we are talking about is between those whose tumors were small (< 1 cm) and larger (>= 2 cm). You can ignore randomization groups for this part of the problem.
Also, a clarification suggested during office hours was to reword the question to ask if it could be due to "lead time" rather than to "lead time bias."
Let me know if this is still not clear.
Best wishes,
Tom