For the stm32l1xx controller, it is indicated everywhere that there are 20 RTC backup registers. Only the first 5 registers work for me. Why?

Question

Microcontrollers

Hello! Question about STM32 controllers, I'm trying to figure out what's wrong.
On the ST website for stm32l1xx series controllers, it is indicated everywhere that they have 20 pieces of 32-bit RTC BACKUP registers that are not reset upon software reset or when going to StandBy mode:
page 547 of the documentation RM0038 Reference manual for STM32L100xx, STM32L151... :
20.6 .20 RTC backup registers (RTC_BKPxR)
Address offset: 0x50 to 0x60 (value line devices)
Address offset: 0x50 to 0x9C (Cat.1 and Cat.2 devices)
Address offset: 0x50 to 0xCC (Cat.3, Cat.4, Cat.5 and Cat.6 devices)
I have an STM32L151CB-A controller and only the first 5 of these registers work in it. However, it has:
DEV_ID = 0x0429
REV_ID = 0x1038
that is, it is a Cat.2 device, why do the other registers not work?
I looked inside the CMSIS sources for STM32L1xx, they define defines for these registers as
follows
: my microcontroller, the stm32l151xba.h file is inserted, it contains the lines:

/******************** Number of backup registers ******************************/
#define RTC_BKP_NUMBER 5

But where is this written in the official documentation?
I found only such a table on the ST website:
https://www.st.com/en/microcontrollers-microproces...
and it only states that controllers without the "-A" prefix contain less kB of RAM, and controllers with prefix "-A" contains more kB of RAM.
What is it? CONSPIRACY? The error of this line of controllers, which is described in some ERRATA and which I did not find on the official website? Or do I just have crooked hands?
Does anyone have any information about this?

Answer 1

It looks like you have A controller. If the Discovery board, then this is found there.
For example, in the datasheet for the controller of the STM32F4-DISCOVERY board it is said that the controller has 192 KB of RAM, in fact - 128. But the information in the datasheet is the subject of change.