Memory usage in AnalogBinLogger of SdFat library

The AnalogBinLogger.ino file of SdFat library says the following:

Each 512 byte data block in the file has a four byte header followed by up to 508 bytes of data. (508 values in 8-bit mode or 254 values in 10-bit mode).
Each block contains an integral number of samples with unused space at the end of the block.

I was wondering why only 254 values can be stored in 10-bit mode instead of (508*8 bits/ 10 bits ~ 406 values ).

1. Is this because of the memory being byte-addressable and hence does a 10-bit value take up 2 bytes?
2. If so, how will a file-reader differentiate between a 10-bit value taking 2 bytes and two 8-bit values?
3. The AnalogBinLogger.ino says:

The logger will use SdFat’s buffer plus BUFFER_BLOCK_COUNT additional buffers.

BUFFER_BLOCK_COUNT is 1 for the Arduino Uno since it has 2KB of SRAM. Each buffer is 512-byte long. Is SdFat’s internal buffer also in SRAM? If I am not wrong, the libraries the program uses and the program code reside in the Flash memory, can SdFat write into a buffer in Flash memory?

// First block of file.
struct metadata_t {
  unsigned long  adcFrequency;     // ADC clock frequency
  unsigned long  cpuFrequency;     // CPU clock frequency
  unsigned long  sampleInterval;   // Sample interval in CPU cycles.
  unsigned long  recordEightBits;  // Size of ADC values, nonzero for 8-bits.
  unsigned long  pinCount;         // Number of analog pins in a sample.
  unsigned long  pinNumber[123];   // List of pin numbers in a sample.
};