That is the question. After working in Telecom for 40 years I have had experience working with many Grounding and Bonding Specs. I got into Telecommunications early enough to work with the pioneers of Bell Labs, and Western Electric. We had a huge conference every year up to the dot com bust in the early 2000s. During those times I learned the origins of Telecom’s 5-ohm spec and NEC 25-ohm spec. Sometimes it helps to know where things came from, and why. For the answer, you have to go back to the US Civil War and the telegraph. My history of the telegraph is not such I can recall model numbers or names but the long line telegraphs required a 5-ohm ground, which allowed up to 450 ohms for the overhead wire conductor. They used dirt as a conductor for one of the battery polarities. Fast forward to 1919 the first rotary dial telephone system was installed in Norfolk Virginia. In the very early days of the telephone was primitive requiring party lines placing up to 12 customers on a single pair of wires leaving the Central Switching Office. So how do you use a single Tip/Ring pair of 20 AWG wire to ring 12 different customer circuits individually with only a 48-volt battery and coffee grinder AC generator? To start the telephone talk circuit is DC, so the ring voltage is AC of roughly 90 to 100-volts. The first thing they did was how they applied Ring Voltage in both Differential and Common Modes. For Differential-Mode, they applied Ring-Voltage Tip-to-Ring. The two Common-Modes were applied Tip-to-Ground and Ring-to-Ground. To use dirt as a conductor for the Ring Circuit required the Central Office Ground to be 5-ohms, and the home to be 25-ohms. That gave the Telco 3 but needed 12. Did I mention Ring Frequencies of 20, 30, 40, and 50 Hz at roughly 90 to 100 volts? They used 4 selective Ring Frequencies x 3 modes = 12 circuits. In those days it required the Telco to come to your house, install the ringer correctly and run a 3-wire circuit of Tip, Ring, and Ground. In those days obtaining a 5 ohm or 25-ohm ground was as easy as falling off a bar stool passing out drunk. Bond to the water pipe and you were done. We were stupid and ignorant in those days. We did not understand how extremely dangerous using ground as a conductor was or how much chaos it creates with RFI and EMI. It took a tall stack of bodies to change things. Seems Plumbers and Water Utility Workers were not happy being shocked and electrocuted. Wives had a problem burying husbands. That is when Water utilities started using dielectric insulators on water meters to stop electric utilities and telephone companies from using water pipes and ground as a circuit conductor. On top of our ignorance, our homes' electrical systems were two-wire systems consisting of Line and Neutral where one of the conductors, neutral is also used as a Ground. Took a lot of bodies and destroyed equipment to change to a 3-wire system with a ground. Today you still see the shadows of the past. Many hams think they need a rod outside the shack, Neutral and Ground (Negative and Ground) are the same things, and AC ground is dirty. All true back in the 50/60’s before transistors and color TV came along when the world was 2-wire. You even see the 25-ohm verbiage in the NEC although is completely meaningless. Paraphrasing NEC 250.53 exception: if a Single Rod does not provide 25-ohms, an additional rod shall be driven. Drive two rods, pass inspection, collect a check, and call it a day. Both rods could be 1000-ohms making a 500 Ohm ground is good enough. You still see the 5-ohm verbiage in some specs for Elmers out there with a Party Line, in a home with 2-wire electrical systems, tube radios, and a grainy Black and White TV set.