Since after all, networks wouldn't be very useful if you could only send pure sine waves (exact frequency) through them.
You have some bandwidth, determined by network type, and acceptable limits of what you're doing; in this case, balance between the two legs, and differential-mode termination.
Roughly speaking, transmission line structures for general (non-precision) purposes have on the order of 10% bandwidth, but this varies from case to case for the above reasons.
LTspice can model it by explicit transmission lines or their LC equivalents.
Tim