Short Answer: Overflow bug.

Long Answer: The formula to calculate the military power of a ship is this:


The reason why the fleet power of the ship is sometimes 300k-700k or even more changes to 1 is that the game usually recalculates it every time your ship engages something, repairs, adds anew ship to fleet, removes a new ship on the fleet, etc. The calculated power of the SB Yamato is sometimes readable but sometimes, is not, See example below.

example:
For now, we'll used a pre-defined value which is stated below:

-Effective health of the yamato is 115k
-DMG per day is 55k

--The Following value below after this are Defined by the Game which is not changable unless modded.
---Exponent is 0.65
---Scale is 0.25

Calculation:

( ( effective_health * damage_per_day ) ^ exponent ) * scale = Power

( ( 115000 x 55000 ) ^ 0.65 ) x 0.25 = 2432692307.692308

The POWER of your SB Yamato Equiped with a Wave Motion Gun is:

2,432,692,307.692308

A ship with 2.4Billion fleet power exceeds the limit of the calculation of Stellaris thus an Overflow happens resulting in a value of 1.

Then why sometimes it changes to a visible number like 700k?
Simple, sometimes, the powerful component on the ship is not included on the calculation, it can be the WMG, the Core, or any other. That's all the reason I could think of on why it jumps from 1 power to 400k power then vice versa.


Side NOTE:
Overflow bug = This is a common bug of stellaris, you can experience this on ships, storage as well as tech cost.
-If your tech cost became free (The repeatable becames free) that's an overflow bug.
-If your storage is full then suddenly EMPTY, that's an overflow bug.