Czech serials

[Canberra TT.18]

Hello,

The Czech serial system used to be straight forward.
4 digits which are in 99% of the cases direct related to the c/n.
But the last couple of years it seems that serials are more given in blocks with no relation to c/n of prevoious serials what so ever.
But there seems to be some logic in the (new) system

Question.
Does anyone know the reason or logic behind this (if there is any)?

examples
serial type c/n
0452 C295M 062
0453 C295M 069
0454 C295M 074
0455 C295M 075

0459 E480B-G 5242
0460 E480B-G 5243

0473 E480B-G 5240
0474 E480B-G 5241
0475 L-39NG 167001 Aero (new)
0476 L-39NG 167004 Aero (new)

0481 C295MW 172
0482 C295MW 173
0484 E480B-G 5230
0485 E480B-G 5245
0486 AH-1Z 59531 c/n?
0487 AH-1Z

0515 E480B-G 5157
0516 E480B-G 5229
0517 L-39NG 167005 for Vietnam


Pieter