I've come across many instances that treat the two terms as separate and distinct. In surveys and polls, Catholics are identified separately from "Christians". More recently, I've noticed that even professional organizations will identify them as separate groups (i.e. mental health counseling). Speaking as a non-Catholic here, I just hope that the distinction is not of malicious intent. One plausible reason is that Catholic expressions of piety (i.e. the rosary) may not be well received by a faithful non-Catholic Christian crowd, and so counseling treatment without consideration of this issue may be offensive to the patient. But still, that doesn't explain why people won't just use terms like "non-Catholic Christian" and "Catholic".
Anyway, regardless of what people believe about the above subject, the main reason for this question is whether this subject of the above paragraph would be on-topic here. It's not really about theology or practice. It's just about whether it's appropriate to classify Catholics as a separate group from so-called "Christians" for professional purpose or academic inquiry.