Yes, I added a source (the first one) which confirms that. 32.5 % of the time the relatives said no, but polls showed that it should only have been 21 %.
Lying is a rather strong word, perhaps they thought that their relative was inclined to refuse donation. And default has no meaning here because they know that their answer decides the outcome; letting it fall to a “default” option decided by others seems to be rather irresponsible to the deceased relative.
Death is a rather traumatic event. Many people won't think straight in this situation. Some may just blurt out "no" because they're in no state to respond to any kind of request.