First, the .TV registry updates its zone about every 6 hours. .COM & the other GTLDs update every 3-5 minutes, so .tv updates will be a bit slower.
Second, your ISP also caches dns values. The length of time they cache does not necessarily (or often) correspond to TTL values, so you may often find that you're getting a cached DNS result from your ISP's DNS resolver, rather than a fresh dns lookup. (the one referred to above that updates every 6 hours)