Why the missing 25 percent of my pings?

Have you tried running a traceroute to see if it gets hung up somewhere. Who is your ISP? ( If you don't mind me asking.)

You connection might be a bit "flaky". It just happened to show up by taking a bigger sample (which took more time) than the smaller sample. When doing such things it's best to repeat these tests a few times to see the average. It may well be that if you did the c4 tests enough times it too might show some packet loss. Error correction handles this stuff but it does slow down your transmission rate somewhat.

Pinging random sites on the internet is highly unreliable. Ping something closer to home. Your ISP is a good start. Nobody can fault you for pinging your own ISP.

Do a traceroute.

traceroute <host name here>

Notice the hops going away from you. Ping each hop until you see where the reliability has fallen off. Also, do more pings per sample - I like 100.

Instead of pinging iad04s01-in-f105.1e100.net as you've tried above, definitely try pinging and trace-routing your ISP as was suggested, and also then try pinging 8.8.8.8 (Google DNS). Google has many public hosts for "google.com". And yes, pinging random hosts might simply show you that the target host or some part of the associated network infrastructure at the remote site is overloaded.

What's your internal network connection? WiFi? Wired? Do you really trust your internal network? Some WiFi networks can feature poor stability; any WiFi in any RF-challenged environments (anywhere with overlapping WiFi installations or with any radio-frequency interference) is suspect. WiFi can sometimes be unstable in a radio-quiet area, and all on its own. I've had a few troubleshooting calls that have traced back to bad patch cables, flaky switches and other LAN-local oddities, too.

Since you asked, I just tried c4, 20, and 100. I get 0% loss in all cases, if that means anything, which I think it doesn't -- different kind of connection (VDSL), different ISP (AT&T U-verse), different location, different DNS (OpenDNS 208.67.222.222, 208.67.220.220), different speed, basically different everything.

The google.com host designation does not designate a single network host, it's a variety of different hosts.

I see five different Google hosts answering round-robin DNS from here, and it would not surprise me to see different hosts geographically. (The hosts I've checked are all Google-domain hosts and not the 1e100.net host domain that you're getting. Though that domain is also using Google DNS servers.)

Further, Google likely shuffles which hosts will respond based on the particular regional network topology.

Instead of pinging iad04s01-in-f105.1e100.net as you've tried above, definitely try pinging and trace-routing your ISP as was suggested, and also then try pinging 8.8.8.8 (Google DNS).

Pinging random hosts might simply show you that the target host or some part of the associated network infrastructure at the remote site is overloaded.

Do you really trust your internal wired network? (You're on really old gear with that Mac, and any technology - DSL or otherwise - can introduce errors.) I've handled networking troubleshooting calls that have traced back to bad patch cables, flaky switches or routers, and other LAN-local oddities, too.

Did you notice you got a different host for Google?