they are more likely to have said she is at risk of a further stroke (for example if the INR is low)
As traci says, inr levels do fluctuate and can depend on what you are eating/drinking, how much exercise you are doing plus are also dependent on you taking the correct dose of warfarin as advised.
As an example, I take it and My target inr is between 2-3 I had a period for 3 years when the dose always remained the same, and the interval between blood tests was extended to 10 weeks. However, on one of the routine BT's my INR had fallen (probably due to a change in diet) and since then its been all over the place, and i'm having to have tests every 7-10 days :(